COUNTERPART, noun [counter and part.] 1. The correspondent part; the part that answers to another, as the two papers of a contract or indentures; a copy; a duplicate. Also, the part which fits another, as the key of a cipher. 2. In music, the part to be applied to another; as, the base is the counterpart to the treble.
Source: Webster's 1828 American Dictionary ↗ · public-domain · returned verbatim, no AI