The 3 undefined terms of geometry are:

  1. Point – A precise location in space with no length, width, or height. It’s usually shown as a dot and named with a capital letter (like point A).
  1. Line – A straight path made of infinitely many points, extending forever in both directions, with no thickness and only one dimension (length).
  1. Plane – A flat, two-dimensional surface that extends infinitely in all directions, containing infinitely many lines and points.

These are called undefined terms because in Euclidean geometry we don’t formally define them using more basic geometric words; instead, we describe them intuitively and then use them as building blocks for all other definitions.