∠, the angle symbol

In geometry, an angle is the figure formed by two rays sharing a common endpoint, called the vertex of the angle. The magnitude of the angle is the "amount of rotation" that separates the two rays, and can be measured by considering the length of circular arc swept out when one ray is rotated about the vertex to coincide with the other (see "Measuring angles", below). Where there is no possibility of confusion, the term "angle" is used interchangeably for both the geometric configuration itself and for its angular magnitude (which is simply a numerical quantity).

Acute (a), obtuse (b), and straight (c) angles. Here, a and b are supplementary angles.

Angles equal to 1/2 turn (180° or two right angles) are called straight angles. (Our solved example in mathguru.com uses this concept)

Two angles that sum to a straight angle (180°) are called supplementary angles.

The difference between an angle and a straight angle (180°) is termed the supplement of the angle.

An angle equal to 1/4 turn (90° or π/2 radians) is called a right angle.

Two lines that form a right angle are said to be perpendicular or orthogonal. (Our solved example in mathguru.com uses this concept)

http://en.wikipedia.org/wiki/Angle

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