Rectilinear angle (geometry)
From Citizendium, the Citizens' Compendium
A rectilinear angle or angle is the "steepness" between two straight lines. The point at which the lines intersect is called the vertex. The parts of the line that extend from the vertex and surround the "steepness" are called the arms. This angle is measured in degrees (parts of 360). This angle can also be measured in radians. There are various special types of angles:

Obtuse angleAn obtuse angle is one where the magnitude of the angle is greater than a right angle, but less than a straight angle. 
Acute angleAn acute angle is one where the magnitude of the angle is less than a right angle. 
Straight angleA straight angle is the angle formed on a straight line. It is equal on both sides of the line. It is equal to the sum of two right angles. 