What math term means opposite?

Answer 1

Reciprocal would do in certain cases.

1 one tenth is the reciprocal of 10 in arithmetic

270º is the reciprocal of 90º in geometry

Inverse would do as well - a reciprocal is an inverse for multiplication.

negation means something like the opposition in logic.

complement has the meaning of opposite in some cases.

opposite is itself a math term- used in geometry as in the phrase "opposite side of a parallellogram".