Adjacent lines are lines that are next to each other in a geometric figure. They share a common point or vertex but do not overlap. In simpler terms, if you have two lines that meet at a point and continue in different directions, they are considered adjacent lines.
For example, in a triangle, each pair of sides are adjacent lines because they meet at a vertex. Similarly, in a rectangle, the sides that share a corner are adjacent lines.
Understanding adjacent lines is important in geometry as it helps in identifying shapes, angles, and various properties of geometric figures.