Principle equivalence refers to the idea that two things have a similar underlying concept or idea, even if they are expressed differently. Synonyms for this term include concept congruence, idea similarity, and principle parity. Other alternatives may include essence equivalence, substance matching, and philosophy consonance. These synonyms all refer to the notion that two things share a fundamental similarity or parallel in their underlying principles or concepts. Whether discussing scientific theories, philosophical concepts, or even everyday ideas, principle equivalence is a key concept for understanding how different expressions or ideas can still share a common foundation.