Perpendicular

Perpendicular – lines that intersect and form 4 right angles

This definition is based on prior knowledge – I remember that perpendicular lines form four 90 degree angles

A line is perpendicular to another if it meets or crosses it at right angles (90°). http://www.mathopenref.com/perpendicular.html

Example: 

http://math.about.com/od/geometry/ss/linessegments_5.htm