Pretty simple concept I learned in my Honors Algebra II/trigonometry, but the term "coterminal angle" doesn't seem to catch on...

Two angles are coterminal when they are in the same position in the Cartesian coordinate plane. For example, the angles 0 and 2π (in radians) are coterminal since they both represent the same position.

Mathematically, if you have the angle θ, then any coterminal angles to θ can be expressed as:

θcoterminal = θ + 2πk

where k is any real integer (positive or negative). (If you're measuring angles in degrees instead of radians, then the term would be "360°k" instead of "2πk".)

One of the properties (I don't think there are many more...) of coterminal angles is that the value of a trig function for angles that are coterminal will be the same for both angles (e.g. since π/6 and 13π/6 are coterminal angles, sin(π/6) = sin(13π/6) = 1/2).

A concept similar to coterminal angles is that of the reference angle.