Definition: An angle whose measure is less than 90°

Try this
Adjust the angle below by dragging an orange dot and see how the angle ∠ABC behaves.
Note that it is acute for all angles from zero to (but not including) 90°

Acute angles are the smallest, being between (but not including) zero and 90°
Note also that acute triangles are those where all the interior angles are acute.

A way to remember

Sometimes we can confuse acute and obtuse angles. A way to remember is that small things tend to be cute.
Acute angle is the smallest type.

Types of angle

Altogether, there are six types of angle as listed below. Click on an image for a full description of that type and a corresponding interactive applet.