• (n.) That point in which the three perpendiculars let fall from the angles of a triangle upon the opposite sides, or the sides produced, mutually intersect.
The point of intersection of the altitudes of a triangle.
Or`tho·cen'ter noun [ Ortho- + center .] (Geom.) That point in which the three perpendiculars let fall from the angles of a triangle upon the opposite sides, or the sides produced, mutually intersect. 
orthocenter The point at which the three altitudes of a triangle intersect.
the point of intersection of the three altitudes of a triangle.