Negative is a term in mathematics that usually means "opposite." An electron's charge is called negative not because it is "below" but because it is opposite that of a proton. A surface with negative curvature bulges in from the point of view of someone on one side of the surface but bulges out from the point of view of someone on the other side. A line with negative slope is downhill for someone moving to the right but uphill for someone moving to the left.

The term negative is most commonly applied to numbers. When negative is an adjective applied to a number or integer, the reference is to the opposite of a positive number. As a noun, negative is the opposite of any given number. Thus, -4, -3/5, and -2 are all negative numbers, but the negative of -4 is +4. The integers, for example, are often defined...