An altitude of any triangle is a segment that extends from a vertex to the opposite side (or an extension of the opposite side) and is perpendicular to that side.  The dashed segments, , in the following figures are altitudes of the triangles:

In the special case of a right triangle, each leg is an altitude perpendicular to the other leg, and there is a third altitude from the right angle perpendicular to the hypotenuse that plays an important role in measurement

Triangle ABC is a right triangle with hypotenuse  and altitude :