To set or place (one thing) over or above something else.
Mathematics To place (one geometric figure) over another so that all like parts coincide.
[Probably French superposer, back-formation from superposition, superposition, from Late Latin superpositiō, superpositiōn-, from Latin superpositus, past participle of superpōnere, to place over : super-, super- + pōnere, to place; see position.]