Noah Webster's New International Dictionary of the English Language

1. (n.) One of two parts that complete each other; anything which answers, or corresponds, to another; a copy; a duplicate; a facsimile.

2. (n.) One of two corresponding copies of an instrument; a duplicate.

3. (n.) A person who closely resembles another.

4. (n.) A thing may be applied to another thing so as to fit perfectly, as a seal to its impression; hence, a thing which is adapted to another thing, or which supplements it; that which serves to complete or complement anything; hence, a person or thing having qualities lacking in another; an opposite.