Define an acute angle.

An acute angle is defined as an angle that measures less than 90 degrees. This characteristic is what distinguishes acute angles from other types of angles, such as right angles, which measure exactly 90 degrees, and obtuse angles, which measure more than 90 degrees but less than 180 degrees. Acute angles play an important role in various geometrical contexts, including triangle properties and trigonometric functions, where understanding their measure is crucial for calculations and theorems. This definition highlights the unique property of acute angles being the smallest type of angle in the classification of angles.