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 obtuse angle is defined as an angle greater than a perpendicular angle.
Degrees are a unit of measure for angles. A full rotation is equal to 360 degrees. In the XY Cartesian Coordinate System, degrees are measured starting from the rightmost edge of the circle.
Radians are a unit that measure angle using the radius of a circle. One radian is equal to the amount of rotation required to travel the length of one radius along the circumference of the circle.

Radians Versus DegreesWumbo (internal)