Triangle properties

Triangle properties

Vertex	The vertex (plural: vertices) is a corner of the triangle. Every triangle has three vertices.
Base	The base of a triangle can be any one of the three sides, usually the one drawn at the bottom. You can pick any side you like to be the base. Commonly used as a reference side for calculating the area of the triangle. In an isosceles triangle, the base is usually taken to be the unequal side.
Altitude	The altitude of a triangle is the perpendicular from the base to the opposite vertex. (The base may need to be extended). Since there are three possible bases, there are also three possible altitudes. The three altitudes intersect at a single point, called the orthocenter of the triangle. See Orthocenter of a Triangle. In the figure above, you can see one possible base and its corresponding altitude displayed.
Median	The median of a triangle is a line from a vertex to the midpoint of the opposite side. The three medians intersect at a single point, called the centroid of the triangle. See Centroid of a Triangle
Area	See area of the triangle and Heron's formula
Perimeter	The distance around the triangle. The sum of its sides. See Perimeter of a Triangle
Interior angles	The three angles on the inside of the triangle at each vertex. See Interior angles of a triangle
Exterior angles	The angle between a side of a triangle and the extension of an adjacent side. SeeExterior angles of a triangle

Also:

1. The shortest side is always opposite the smallest interior angle
2. The longest side is always opposite the largest interior angle