1. The act or process of equating or of being equated.

2. The state of being equal.

3. Mathematics A statement asserting the equality of two expressions, usually written as a linear array of symbols that are separated into left and right sides and joined by an equal sign.

4. Chemistry A representation of a chemical reaction, usually written as a linear array in which the symbols and quantities of the reactants are separated from those of the products by an arrow or a set of opposing arrows.

5. A complex of variable elements or factors: "The world was full of equations ... there must be an answer for everything, if only you knew how to set forth the questions" (Anne Tyler).