Mirroring is an automatic motor and expressive echo; empathy is understanding what another person is experiencing. Mirroring can support empathy and often accompanies it, but it runs without it — you can copy someone's posture perfectly while understanding nothing about them.
Side by side
Aspect
Mirroring
Empathy
Is
Behavioural echo
Understanding
Conscious?
Rarely
Often
Requires the other
Present
Not necessarily
Can be faked
Yes, badly
Harder
What each one means
Mirroring
Mirroring is the largely unconscious copying of another person's gestures, expressions, posture or speech during interaction — and, in a separate therapeutic sense, the attuned reflecting-back that helps a person build a stable sense of self.
Understanding what another person feels, and often feeling it with them while still knowing the feeling is theirs. It began as a term in German aesthetics.