right angle 
noun
An angle of 90°, as in a corner of a square, or formed by dividing a circle into quarters.It follows that on the earth's surface there is an equilateral triangle with all its angles equal to a right angle....
- Give to a philosopher the concept of a triangle, and let him discover in his own way what the relation of the sum of its angles to a right angle might be.
- He also designed an instrument to divide a right angle into a given number of equal parts.
Phrases
at right angles (or a right angle) to