An angle is defined as the amount of rotation between two rays. The point where the rays start is called the vertex. There are two systems for measuring angles in mathematics: degrees and radians. While the degree angle system is often used to introduce concepts in math, the radian system eventually becomes the preferred unit for measuring angles^{[1]}.
A perpendicular angle is equal to a quarter of a full rotation.
An acute angle is defined as an angle smaller than a perpendicular angle.
An angle greater than 90 degrees.

Radians Versus DegreesWumbo (internal)