late 14c., a term in astrology; meaning "action of making equal" is from 1650s; mathematical sense is from 1560s, on notion of equalizing the expressions; from Latin aequationem (nominative aequatio) "an equal distribution, community," from past participle stem of aequare (see equal (adj.)). Chemistry sense is from 1807.
equation e·qua·tion (ĭ-kwā'zhən, -shən)
A statement asserting the equality of two mathematical expressions, usually written as a linear array of symbols that are separated into left and right sides and are joined by an equal sign.
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 equal sign, an arrow, or a set of opposing arrows.
An expression of equality between two formulas in mathematics. The two formulas are written with an equal sign between them: 2 + 2 = 4 is an equation, as is E = mc2.