noun

• The quality of agreeing or corresponding; being suitable and appropriate.
• (mathematics, number theory) A relation between two numbers indicating they give the same remainder when divided by some given number.
• (mathematics, geometry) The quality of being isometric — roughly, the same measure and shape.
• (mathematics, linear algebra) Matrix similarity by an orthogonal matrix.
• (algebra, more generally) Any equivalence relation defined on an algebraic structure which is preserved by operations defined by the structure.