means The quality of being made up of balanced, matching parts — where one half mirrors the other, or a pattern stays the same when you flip, rotate, or shift it.

from From Greek 'symmetria,' meaning 'agreement in measure' — built from 'syn-' (together) and 'metron' (measure), the same 'metron' that gives us 'meter.' So at its root, symmetry literally means things 'measuring together,' in proportion. It traveled through Latin 'symmetria' and into English in the 1500s, first describing harmonious proportions in art and architecture before mathematicians and physicists claimed it for their own.