A mirror is a polished surface that reflects a clear image of whatever stands before it — and, by extension, anything that gives a faithful likeness or representation of something else.
The word reached English around the mid-thirteenth century as mirour, from Old French mireoir, "reflecting glass; observation, model, example," which traces through Vulgar Latin back to Latin mirari, "to wonder at, to admire" — the same root behind miracle and admire. The figurative sense followed almost immediately: by about 1300 a mirror was already something that exemplifies or instructs, a usage preserved in the medieval "mirror for princes," a genre of treatises holding up model conduct for a ruler to copy. The verb sense, "to reflect," is later, dating to the 1590s.
Modern uses fan out from that core. In computing, a mirror is a server holding a replicated copy of another site's data. In psychology, mirroring names the largely unconscious imitation of another person's posture, speech, or affect — behavior that means something quite different in clinical contexts, where 📝Mirroring describes the adoption of a partner's tastes and identity as one's own.
The distinction the word rewards holding is between reflection and reversal. A mirror does not reproduce; it transposes. What it returns is congruent with the original and never identical to it, which is why handwriting held up to one reads only backwards — and why "mirror" as a metaphor for another person, as in 📝The Mirror They Couldn't Handle, carries an implication of confrontation rather than mere resemblance.