Definition
Perpendicular lines
Perpendicular lines intersect to make right angles at their meeting point.
What it means
Picture the corner of a sheet of paper extended in both directions. One line runs along one edge and the other along the next edge. They cross once, and every angle around the crossing is a right angle.
Remember it
Think of a plus-sign crossing with a tiny square corner mark.
Example
The horizontal line y = 2 and the vertical line x = 5 meet at (5, 2). Their directions form a 90° angle, so the lines are perpendicular.
Watch for
Lines that look nearly square are not enough; the angle at the intersection must be exactly 90°.
Common mix-up
Parallel lines stay the same distance apart and never meet; perpendicular lines meet at right angles.
Exact meaning
In plane geometry, two lines are perpendicular when they intersect and an angle between them measures 90°. The other three angles at that intersection are then also 90°.