What is the primary characteristic of an acute angle?

An acute angle is defined as an angle that measures less than 90 degrees. This characteristic distinctly sets acute angles apart from other types of angles. For example, a right angle measures exactly 90 degrees, which means it is neither acute nor obtuse. An obtuse angle measures more than 90 degrees but less than 180 degrees, which again distinguishes it from acute angles. Furthermore, angles that lie between 90 and 180 degrees are classified as obtuse angles, reinforcing that acute angles are specifically those that measure less than 90 degrees. Thus, the primary characteristic of an acute angle is its measure being always less than 90 degrees, making the choice accurate in identifying the nature of acute angles.