An angle is the amount of turn between two straight lines that have a common end point or vertex. In geometry, there are five types of angles. An obtuse angle is greater than 90 degrees but less than 180 degrees. An acute angle is less than 90 degrees, A right angle is exactly 90 degrees. A straight angle is exactly 180 degrees, and a reflex angle is greater than 180 degrees. A full angle, measuring 360 degrees, may be considered a sixth type of angle. In these terms, another way of defining an obtuse angle is an angle whose measure is greater than a right angle and less than a straight angle.

For those who find the different angle definitions confusing or difficult to remember, the following is a common strategy for remembering: acute angles are "cute" or small; obtuse angles are "obtuse," fat or wide; right angles are "right" as in correct or perfect; straight angles are "straight" like a line and reflex angles can be thought to have good "reflexes," as in being bendy or overstretched.