Either of two parts that fit together, or complement one another.

(law) A duplicate of a legal document.

One who or that which resembles another. (Can we add an example for this sense?)

One who or that which has corresponding functions or characteristics.

(paleontology) Either half of a flattened fossil when the rock has split along the plane of the fossil.

(transitive) To counterbalance. (Can we add an example for this sense?)