Emily Dickinson Lexicon

E-QUA'TION, n. [L. æquatio, from æquo, to make equal or level.]

1. Literally, a making equal, or an equal division.
2. In algebra, a proposition asserting the equality of two quantities, and expressed by the sign = between them; or an expression of the same quantity in two dissimilar terms, but of equal value, as 3s=36d, or x=b+m-r. In the latter case, x is equal to b added to m, with r subtracted, and the quantities on the right hand of the sign of equation are said to be the value of x on the left hand. Encyc. Johnson.
3. In astronomy, the reduction of the apparent time or motion of the sun to equable, mean, or true time. Encyc.
4. The reduction of any extremes to a mean proportion. Harris.