What does "inverse" mean?

Additional senses

1. 2.(Math.) Opposite in nature and effect; -- said with reference to any two operations, which, when both are performed in succession upon any quantity, reproduce that quantity; as, multiplication is the inverse operation to division. The symbol of an inverse operation is the symbol of the direct operation with -1 as an index. Thus sin-1 x means the arc whose sine is x. Inverse figures (Geom.), two figures, such that each point of either figure is inverse to a corresponding point in the order figure. -- Inverse points (Geom.), two points lying on a line drawn from the center of a fixed circle or sphere, and so related that the product of their distances from the center of the circle or sphere is equal to the square of the radius. -- Inverse, or Reciprocal, ratio (Math.), the ratio of the reciprocals of two quantities. -- Inverse, or Reciprocal, proportion, an equality between a direct ratio and a reciprocal ratio; thus, 4 : 2 : : , or 4 : 2 : : 3 : 6, inversely.
2. 3.That which is inverse. Thus the course of human study is the inverse of the course of things in nature. Tatham.