Having two sides, bilateral. / Reversible. / (algebra) Whose salient properties apply when (the specified object) appears on either side of a given binary operator.

(algebra) A subring which is both a left ideal and a right ideal. Commonly referred to simply as an ideal, but referred to as two-sided ideal for emphasis.

plural of two-sided ideal

One should not make a judgement until one hears the other side.

Two aspects of the same thing. / Two things that superficially appear to be opposites but are fundamentally similar.