diff --git a/README-extentions.rst b/README-extentions.rst
index 8223348..ec10e48 100644
--- a/README-extentions.rst
+++ b/README-extentions.rst
@@ -124,19 +124,19 @@
     three: ${one}
 
 
-Fixed/Immutable Parameters
+Constant Parameters
 --------------------------
 
-Parameters can be labeled as fixed or immutable by using the prefix ``=``
+Parameters can be labeled as constant by using the prefix ``=``
 
 .. code-block:: yaml
 
   parameters:
     =one: 1
 
-If in the normal parameter merging a fixed parameter would be changed then depending
-on the setting of ``ignore_merging_onto_fixed`` either an exception is raised (``ignore_merging_onto_fixed`` false)
-or the parameter is left unchanged and no notification or error is given (``ignore_merging_onto_fixed`` true)
+If in the normal parameter merging a constant parameter would be changed then depending
+on the setting of ``strict_constant_parameters`` either an exception is raised (``strict_constant_parameters`` true)
+or the parameter is left unchanged and no notification or error is given (``strict_constant_parameters`` false)
 
 For example with:
 
@@ -155,8 +155,8 @@
   parameters:
     one: 2
 
-``reclass.py --nodeinfo node1`` then gives an ''Attempt to change fixed value'' error if ``ignore_merging_onto_fixed``
-is false or gives:
+``reclass.py --nodeinfo node1`` then gives an ''Attempt to change constant value'' error if ``strict_constant_parameters``
+is true or gives:
 
 .. code-block:: yaml
 
@@ -164,13 +164,13 @@
     alpha:
       one: 1
 
-if ``ignore_merging_onto_fixed`` is true
+if ``strict_constant_parameters`` is false
 
-Default value for ``ignore_merging_onto_fixed`` is False
+Default value for ``strict_constant_parameters`` is True
 
 .. code-block:: yaml
 
-  ignore_merging_onto_fixed: False
+  strict_constant_parameters: True
 
 
 Nested References
diff --git a/reclass/datatypes/parameters.py b/reclass/datatypes/parameters.py
index e40ded1..fa0f379 100644
--- a/reclass/datatypes/parameters.py
+++ b/reclass/datatypes/parameters.py
@@ -183,8 +183,8 @@
                     value = Value(value, self._settings, self._uri, parse_string=self._parse_strings)
                 if key[0] == self._settings.dict_key_override_prefix:
                     value.overwrite = True
-                elif key[0] == self._settings.dict_key_fixed_prefix:
-                    value.fixed = True
+                elif key[0] == self._settings.dict_key_constant_prefix:
+                    value.constant = True
                 value = self._merge_recurse(cur.get(newkey), value)
                 key = newkey
             else:
diff --git a/reclass/datatypes/tests/test_parameters.py b/reclass/datatypes/tests/test_parameters.py
index 013bd88..5959197 100644
--- a/reclass/datatypes/tests/test_parameters.py
+++ b/reclass/datatypes/tests/test_parameters.py
@@ -20,7 +20,7 @@
 from reclass.utils.parameterdict import ParameterDict
 from reclass.values.value import Value
 from reclass.values.scaitem import ScaItem
-from reclass.errors import ChangedFixedError, InfiniteRecursionError, InterpolationError, ResolveError, ResolveErrorList, TypeMergeError
+from reclass.errors import ChangedConstantError, InfiniteRecursionError, InterpolationError, ResolveError, ResolveErrorList, TypeMergeError
 import unittest
 
 try:
@@ -755,18 +755,18 @@
         p1.interpolate()
         self.assertEqual(p1.as_dict(), r)
 
-    def test_fixed_parameter(self):
+    def test_strict_constant_parameter(self):
         p1 = Parameters({'one': { 'a': 1} }, SETTINGS, 'first')
         p2 = Parameters({'one': { '=a': 2} }, SETTINGS, 'second')
         p3 = Parameters({'one': { 'a': 3} }, SETTINGS, 'third')
-        with self.assertRaises(ChangedFixedError) as e:
+        with self.assertRaises(ChangedConstantError) as e:
             p1.merge(p2)
             p1.merge(p3)
             p1.interpolate()
-        self.assertEqual(e.exception.message, "-> \n   Attempt to change fixed value, at one:a, in second; third")
+        self.assertEqual(e.exception.message, "-> \n   Attempt to change constant value, at one:a, in second; third")
 
-    def test_fixed_parameter_allow(self):
-        settings = Settings({'ignore_merging_onto_fixed': True})
+    def test_constant_parameter(self):
+        settings = Settings({'strict_constant_parameters': False})
         p1 = Parameters({'one': { 'a': 1} }, settings, 'first')
         p2 = Parameters({'one': { '=a': 2} }, settings, 'second')
         p3 = Parameters({'one': { 'a': 3} }, settings, 'third')
diff --git a/reclass/defaults.py b/reclass/defaults.py
index afae213..1e50c0e 100644
--- a/reclass/defaults.py
+++ b/reclass/defaults.py
@@ -29,7 +29,7 @@
 OPT_IGNORE_CLASS_NOTFOUND_WARNING = True
 
 OPT_IGNORE_OVERWRITTEN_MISSING_REFERENCES = True
-OPT_IGNORE_MERGING_ONTO_FIXED = False
+OPT_STRICT_CONSTANT_PARAMETERS = True
 
 OPT_ALLOW_SCALAR_OVER_DICT = False
 OPT_ALLOW_SCALAR_OVER_LIST = False
@@ -51,7 +51,7 @@
 EXPORT_SENTINELS = ('$[', ']')
 PARAMETER_INTERPOLATION_DELIMITER = ':'
 PARAMETER_DICT_KEY_OVERRIDE_PREFIX = '~'
-PARAMETER_DICT_KEY_FIXED_PREFIX = '='
+PARAMETER_DICT_KEY_CONSTANT_PREFIX = '='
 ESCAPE_CHARACTER = '\\'
 
 AUTOMATIC_RECLASS_PARAMETERS = True
diff --git a/reclass/errors.py b/reclass/errors.py
index 1a3790c..0c9d48f 100644
--- a/reclass/errors.py
+++ b/reclass/errors.py
@@ -306,13 +306,13 @@
         return msg
 
 
-class ChangedFixedError(InterpolationError):
+class ChangedConstantError(InterpolationError):
 
     def __init__(self, uri):
-        super(ChangedFixedError, self).__init__(msg=None, uri=uri, tbFlag=False)
+        super(ChangedConstantError, self).__init__(msg=None, uri=uri, tbFlag=False)
 
     def _get_error_message(self):
-        msg = [ 'Attempt to change fixed value' + self._add_context_and_uri() ]
+        msg = [ 'Attempt to change constant value' + self._add_context_and_uri() ]
         return msg
 
 
diff --git a/reclass/settings.py b/reclass/settings.py
index 01166a9..51c518f 100644
--- a/reclass/settings.py
+++ b/reclass/settings.py
@@ -23,15 +23,15 @@
         self.default_environment = options.get('default_environment', DEFAULT_ENVIRONMENT)
         self.delimiter = options.get('delimiter', PARAMETER_INTERPOLATION_DELIMITER)
         self.dict_key_override_prefix = options.get('dict_key_override_prefix', PARAMETER_DICT_KEY_OVERRIDE_PREFIX)
-        self.dict_key_fixed_prefix = options.get('dict_key_fixed_prefix', PARAMETER_DICT_KEY_FIXED_PREFIX)
-        self.dict_key_prefixes = [ str(self.dict_key_override_prefix), str(self.dict_key_fixed_prefix) ]
+        self.dict_key_constant_prefix = options.get('dict_key_constant_prefix', PARAMETER_DICT_KEY_CONSTANT_PREFIX)
+        self.dict_key_prefixes = [ str(self.dict_key_override_prefix), str(self.dict_key_constant_prefix) ]
         self.escape_character = options.get('escape_character', ESCAPE_CHARACTER)
         self.export_sentinels = options.get('export_sentinels', EXPORT_SENTINELS)
         self.inventory_ignore_failed_node = options.get('inventory_ignore_failed_node', OPT_INVENTORY_IGNORE_FAILED_NODE)
         self.inventory_ignore_failed_render = options.get('inventory_ignore_failed_render', OPT_INVENTORY_IGNORE_FAILED_RENDER)
         self.reference_sentinels = options.get('reference_sentinels', REFERENCE_SENTINELS)
         self.ignore_class_notfound = options.get('ignore_class_notfound', OPT_IGNORE_CLASS_NOTFOUND)
-        self.ignore_merging_onto_fixed = options.get('ignore_merging_onto_fixed', OPT_IGNORE_MERGING_ONTO_FIXED)
+        self.strict_constant_parameters = options.get('strict_constant_parameters', OPT_STRICT_CONSTANT_PARAMETERS)
 
         self.ignore_class_notfound_regexp = options.get('ignore_class_notfound_regexp', OPT_IGNORE_CLASS_NOTFOUND_REGEXP)
         if isinstance(self.ignore_class_notfound_regexp, string_types):
@@ -56,7 +56,7 @@
                and self.default_environment == other.default_environment \
                and self.delimiter == other.delimiter \
                and self.dict_key_override_prefix == other.dict_key_override_prefix \
-               and self.dict_key_fixed_prefix == other.dict_key_fixed_prefix \
+               and self.dict_key_constant_prefix == other.dict_key_constant_prefix \
                and self.escape_character == other.escape_character \
                and self.export_sentinels == other.export_sentinels \
                and self.inventory_ignore_failed_node == other.inventory_ignore_failed_node \
@@ -65,7 +65,7 @@
                and self.ignore_class_notfound == other.ignore_class_notfound \
                and self.ignore_class_notfound_regexp == other.ignore_class_notfound_regexp \
                and self.ignore_class_notfound_warning == other.ignore_class_notfound_warning \
-               and self.ignore_merging_onto_fixed == other.ignore_merging_onto_fixed
+               and self.strict_constant_parameters == other.strict_constant_parameters
 
     def __copy__(self):
         cls = self.__class__
diff --git a/reclass/values/value.py b/reclass/values/value.py
index ffb116f..4e86274 100644
--- a/reclass/values/value.py
+++ b/reclass/values/value.py
@@ -24,7 +24,7 @@
         self._settings = settings
         self._uri = uri
         self._overwrite = False
-        self._fixed = False
+        self._constant = False
         if isinstance(value, string_types):
             if parse_string:
                 try:
@@ -50,12 +50,12 @@
         self._overwrite = overwrite
 
     @property
-    def fixed(self):
-        return self._fixed
+    def constant(self):
+        return self._constant
 
-    @fixed.setter
-    def fixed(self, fixed):
-        self._fixed = fixed
+    @constant.setter
+    def constant(self, constant):
+        self._constant = constant
 
     @property
     def uri(self):
diff --git a/reclass/values/valuelist.py b/reclass/values/valuelist.py
index aa7ac70..9c1e1fa 100644
--- a/reclass/values/valuelist.py
+++ b/reclass/values/valuelist.py
@@ -13,7 +13,7 @@
 import copy
 import sys
 
-from reclass.errors import ChangedFixedError, ResolveError, TypeMergeError
+from reclass.errors import ChangedConstantError, ResolveError, TypeMergeError
 
 
 
@@ -48,7 +48,7 @@
         self._is_complex = False
         item_type = self._values[0].item_type()
         for v in self._values:
-            if v.is_complex() or v.fixed or v.overwrite or v.item_type() != item_type:
+            if v.is_complex() or v.constant or v.overwrite or v.item_type() != item_type:
                 self._is_complex = True
 
     def has_references(self):
@@ -109,7 +109,7 @@
         output = None
         deepCopied = False
         last_error = None
-        fixed = False
+        constant = False
         for n, value in enumerate(self._values):
             try:
                 new = value.render(context, inventory)
@@ -123,11 +123,11 @@
                 else:
                     raise e
 
-            if fixed:
-                if self._settings.ignore_merging_onto_fixed:
-                    continue
+            if constant:
+                if self._settings.strict_constant_parameters:
+                    raise ChangedConstantError('{0}; {1}'.format(self._values[n-1].uri, self._values[n].uri))
                 else:
-                    raise ChangedFixedError('{0}; {1}'.format(self._values[n-1].uri, self._values[n].uri))
+                    continue
 
             if output is None or value.overwrite:
                 output = new
@@ -179,8 +179,8 @@
                         output = new
                         deepCopied = False
 
-            if value.fixed:
-                fixed = True
+            if value.constant:
+                constant = True
 
         if isinstance(output, (dict, list)) and last_error is not None:
             raise last_error