What is an acute angle?

When two rays AO and BO meet at a common point  (or OA and
OB start drom a common point O), then there are two angles AOB at O: Smaller angle AOB
(or BOA) and larger angle AOB( or BOA) at O.

An angle is
the measure of the inclination of two rays with a common  point both
rays..

The angle between the opposite rays is 180 degrees 
or pi radians, or 2right angles.

If a ray stands on another
ray , then there are two angles adding to two right angles(or 180 degrees , or pi
radians).

A ray is said to make  right angles when it
stands on another ray if the the two angles it makes on the other ray are equal in
measure.

If the measure of angle between the two rays
meeting at a common point or starting from a common point is less than the right angle
(90 degrees, or pi/2 radians), then the angle is said to be
acute.