(Jimmy of the Clue Crew shows an animated angle on the monitor.)  With a measure of less than 90 degrees, an angle like the one seen here is referred to as this type of angle
In One Diagram 
... And the two straight sides are called arms ... 1. give the angle a name, usually 
a lower-case letter like a or b, or sometimes a Greek letter like  (alpha) or  (
theta).