In triangle geometry, an altitude is a line from a vertex perpendicular to the opposite side. It is an example of a Cevian line. The three altitudes are concurrent, meeting in the orthocentre. The feet of the three altitudes form the orthic triangle (which is thus a pedal triangle), and lie on the nine-point circle. The area of the triangle is equal to half the product of an altitude and the side it meets.
- H.S.M. Coxeter; S.L. Greitzer (1967). Geometry revisited. MAA. ISBN 0-88385-619-0.