A line is a geometric figure that extends infinitely in both directions. In diagrams, it is usually drawn as a straight path with arrowheads at both ends, showing that it has no endpoints.
A line is determined by any two distinct points. In coordinate geometry, a line can be written in forms such as y = mx + b, where m represents its slope and b its y-intercept. A vertical line is commonly written x = a because its slope is undefined.
Students often confuse a line with a line segment or a ray. A segment has two endpoints and a finite length. A ray begins at one endpoint and continues forever in one direction. A line has neither endpoint and continues endlessly in both directions. In elementary Euclidean geometry, “line” describes an ideal object rather than a physical mark, since every visible drawing has a limited length.