1. An altitude of a triangle

  1. An altitude is a line which passes through a vertex of a triangle, and meets the opposite side at right angles. A triangle has three altitudes. For more see Altitudes of a triangle.

    An interesting fact is that the three altitudes always pass through a common point called the orthocenter of the triangle. See Orthocenter of a triangle

  2. The length of a perpendicular from a side of the triangle to the opposite vertex. This is often used to calculate the area of a triangle. See Area of a triangle.

2. Quadrilaterals with a pair of parallel sides

If a quadrilateral has a pair of parallel sides, both of them are called a base. Base and altitude of a parallelogram In a similar way to triangles, the altitude of such a figure is the perpendicular distance from a base to the opposite side. Since they are parallel, either one will do.

Note: A common mistake is to use the length of the slanted side as the altitude. This is wrong. You must use the vertical distance as shown.