A line is straight, has no thickness, and extends forever in both directions. It is one-dimensional: only length matters. Two points determine a unique line. In Euclidean geometry, the line is one of the undefined terms, described by axioms rather than defined by simpler objects.

Lines can be parallel, intersecting, or coincident. Parallel lines never meet and have the same slope. Perpendicular lines meet at right angles, and their slopes are negative reciprocals. A line segment is a finite part of a line between two points; a ray extends from one point infinitely in one direction.

In analytic geometry, a line is described by equations like y = mx + b or Ax + By = C. In higher dimensions, lines are defined parametrically. Lines appear in graphs, blueprints, and coordinate systems. The concept is simple but foundational to geometry and algebra.

• Straight, one-dimensional, infinite
• Determined by two points
• Parallel, intersecting, or perpendicular
• Described by equations in analytic geometry