In geometry, a line is a perfectly straight one-dimensional figure extending infinitely in both directions. There are two subsets, or subcategories, of lines in geometry: line segments and rays.

A line segment is a piece of a line that has two distinct endpoints. Because of these endpoints, unlike a line, a line segment doesn't extend infinitely. Rather, it is finite, with a measurable length.

A ray is essentially a hybrid between a line and a line segment. A ray has exactly one endpoint -- called its origin -- and extends infinitely in the other direction. Like lines, rays are infinite and therefore immeasurable. Rays may sometimes be referred to as half-lines.