Angle
An angle is defined as the amount of rotation between two rays. There are two systems for measuring angles in mathematics: degrees and radians. A full rotation in degrees is . A full rotation in radians is (tau) radians or approximately radians. For higher level math, radians are the preferred system to measure angles^{[1]}.
An acute angle is an angle that is smaller than 90 degrees or PI fourths.
An obtuse angle is an angle that is larger than 90 degrees or PI fourths.
A Perpendicular angle, sometimes also referred to as a square angle, is exactly 90 degrees or PI fourths.
Complementary angles can visually be denoted as two angles who sum to a perpendicular or square angle.
Supplementary angles can visually be denoted as two angles who sum to 180 degrees or PI degrees.
The degree angle system divides a full rotation into degrees.
The radian angle system is used in higher level math to replace the degree angle system. A full rotation in radians is equal to (tau) radians.
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.
The circle constant τ (tau) is a geometric constant approximately equal to 6.283. The numeric value is defined as the length of any circle's circumference divided by the length of its radius.

Radians Versus DegreesWumbo (internal)