Altitude of a Triangle

The distance between a vertex of a triangle and the opposite side is altitude. Formally, the shortest line segment between a vertex of a triangle and the (possibly extended) opposite side.

Note: Every triangle have 3 altitudes which are intersecting at one point - orthocenter. If we use the standard notations, in a triangle ABC, there are three altitudes: AH_a, BH_b, CH_c. The three lines meet at a point - the orthocenter (H at the picture) of the triangle. For an obtuse triangle (having one angle exceeding 90°), the orthocenter lies outside the triangle, because the lines of the altitudes intersect in one point.