noun
A mathematical equation that holds true for all values of its variables. Used in algebra and other branches of mathematics.
この式は恒等式なので、xにどんな値を代入しても成り立つ。
Since this expression is an identity, it holds true no matter what value you substitute for x.
方程式 is an equation that holds only for specific values of the variables, while 恒等式 holds for all values.
Composed of 恒 (constant, always) + 等 (equal) + 式 (formula, expression). The term is a direct calque from Western mathematical terminology.