An angle is named using three letters, where the middle letter corresponds to the vertex of the angle. The angle at the right may be referred to as ∠ABC or ∠CBA. If it is perfectly clear which angle is being named, an angle may be referred to by its vertex letter alone, such as, in this case, ∠B. (See more about naming angles.)
It is understood, unless otherwise stated, that angles are positive, counterclockwise, and less than or equal to 180º.

An acute angle is an angle whose measure is less than 90º.

A right angle is an angle whose measure is 90º.

A straight angle is an angle whose measure is 180º.

A reflex angle is an angle whose measure is greater than 180º but less than 360º
If an angle is a reflex angle, it will be clearly indicated by a drawing or by a reference in the wording of the problem.

An oblique angle is any angle that is not 90º, not a right angle.

A dihedral angle is an angle between two planes.