1. altitude of a triangle a point in which the three altitudes of the triangle intersect. it is possible for the orthocenter to occur on the triangle, on the interior of the triangle or on the exterior of the triangle. 2. base angles the third, unequal side of an isosceles triangle. 3. base of an isosceles triangle a segment from a vertex to the midpoint of the opposite side. 4. centroid the two angles that include the base of an isosceles triangle. 5. median of a triangle a point on the interior of a triangle in which the three medians of the triangle intersect. 6. orthocenter a segment from a vertex perpendicular to the opposite side. 7. vertex angle the angle formed by the two equal sides of an isosceles triangle.