(adj) such that the terms of an expression cannot be interchanged without changing the meaning, Example: the arguments of the symmetric relation, `is the father of', are noninterchangeable
adj. 1. (math, logic) not able to be interchanged without changing the truth value. [ WordNet 1.5 ]
2. Not able to be substituted for each other; -- of objects, such as parts of machinery; as, the ink cartidges on the new printer are noninterchangeable with those on the older model. [ PJC ]