Definition
Acute angle
An acute angle has a measure greater than 0° and less than 90°.
What it means
Start with two rays together and open them, but stop before the opening reaches a square corner. The way the angle points does not matter; only the size of the opening matters.
Remember it
Acute stops before the square corner.
Example
An angle measures 35°. Because 0° < 35° < 90°, the angle is acute.
Watch for
A 0° angle is not acute, and a 90° angle is a right angle rather than an acute angle.
Common mix-up
Acute is under 90°; obtuse is over 90° but under 180°.
Exact meaning
An acute angle is an angle whose measure is strictly between 0° and 90°. Rotating the angle or lengthening its rays does not change its classification.