noun.
The state of being invariable; constancy of state, condition, or quality; immutability; unchangeableness.
WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
noun.
the quality of being resistant to variation
Word Usage
"The idea of causality, which implies the universal necessity of causation, cannot be educed from experience, since necessity (as opposed to mere invariableness) cannot be known by experience; it is therefore an original principle in the mind."