Basic geometric terms

A list of the basic geometric terms that other articles assume you know.

List

• A point is the most basic geometric idea. It is a geometric object with no size, and we typically use it to define other geometric objects. In a plane, a point is a pair of numbers giving its coordinates. In space we need three coordinates, so a point is a triple. We usually write a point with round brackets, like this: (1, 4), and name it with a single capital letter, for example point B. A point is usually marked with a cross or a filled dot.

• A line segment is marked out by two endpoints joined by a straight line. We name a line segment by its endpoints, so we can talk about line segment AB if its endpoints are A and B. We can measure the length of a line segment, which is the distance between its endpoints. When we talk about the length of line segment AB, we write it with vertical bars: |AB|. The next figure shows three different line segments: AB, CD and EF. The length of line segment AB is three, that is, |AB| = 3.