regular & irregular, actinomorphic & zygomorphic, symmetry.

A regular flower is one in which all members of each set of flowering parts (stamens, pistils, petals, sepals) are alike in regard to form, orientation and colour.¹ In reference to flowers, the terms radial or radial symmetry (divisible into mirror-image halves in three or more ways²) and actinomorphic are synonymous with regular.

In irregular flowers, there are inequalities between members within the same set. The terms bilateral or bilateral symmetry (divisible into mirror-image halves in only one way²) and zygomorphic are synonymous with irregular.

In addition to radial and bilateral symmetry, individual structures might be characterized by biradial symmetry (divisible into mirror-image halves in two ways²) or asymmetry (not divisible into identical or mirror-image halves²). Some authors do not distinguish between radial and biradial symmetry, while others do.³