in a way that involves having two parts that match exactly, either when one half is like an image of the other half in a mirror, or when one part can take the place of another if it is turned 90° or 180°

The specimen consists of about thirty-five symmetrically developed crystals. 该标本由大约 三十五个对称发育的晶体组成。

in a way that involves actions or parts that are similar or balanced in some way

His book is less traditional and is less symmetrically balanced. 他的书不循传统，也无四平八稳的结构。