Angle is widely used in geometry and trigonometry.
Euclid, the father of geometry, once defined an angle as the relative inclination of two non-parallel straight lines in a plane. Proclos thinks that angle may be a trait, a quantifiable quantity, or a relationship. Oldham thinks that an angle is a deviation from a straight line, and Cabus of Antioch thinks that an angle is a space between two intersecting straight lines. Euclid thinks that an angle is a relationship, but his definitions of right angle, acute angle or obtuse angle are quantitative.
Extended data
1, zero degree angle
The angle is equal to 0, or a line.
2. acute angle
Angle greater than 0 and less than 90, or radian greater than 0 and less than {\displaystyle \pi /2}.
Step 3: Right angle
The angle is equal to 90, or the angle with radian of {\displaystyle \pi /2}.
4, obtuse angle
The angle is greater than 90 and less than 180, or the radian is greater than {\displaystyle \pi /2} and less than {\displaystyle \pi}.
5. Boxer angle
The angle is equal to 180, or the angle with radian of {\displaystyle \pi}.
6. Dominant angle or dihedral angle
The angle is greater than 180 and less than 360, or the radian is greater than {\displaystyle \pi} and less than {\displaystyle 2\pi}.
7. round angle
The angle is equal to 360, or the angle with radian of {\displaystyle 2\pi}.
Refer to Baidu Encyclopedia.