Simple Definition:

Perpendicular means that two lines or surfaces meet at a 90-degree angle (a right angle). This means that they are straight, and they intersect each other at a right angle. For example, if you have a horizontal line and a vertical line, they are perpendicular to each other.

Very Simple Definition:

Perpendicular means two lines or surfaces that meet to form a corner (like the corner of a square). For example, the wall of a house and the floor of the house form a corner, and they are perpendicular to each other.