Angle is the geometric concept you learn after the line, line segment and ray. Indeed the idea of angle opens up and leads to more ideas and concepts and thus contributes to the expansion of the subject. An angle is formed by two intersecting rays at the point of intersection. The rays are called the arms or sides of the angle and the point of intersection is called the vertex of the angle.

An acute angle is an angle whose degree measure is greater than 0 but less than 90.
In general angle if angle A is acute its degree measure < 90º.

Acute Angle

In the above diagram angle 1 is seen smaller than the right angle marked by a square sign. Hence it is an acute angle.

Similarly, the angle marked as 2 is called the complementary angle of angle 1. The measures of angles 1 and 2 add up to 90º.
Hence the complementary angle of an acute angle is also acute as it measures less than 90º.
In the adjoining diagram angle 1 is formed at the intersection of the rays AB and AC.
AB and AC are called the arms or sides of the angles. A is the vertex where the angle is formed.
The angle is denoted in any of the following manner.
∠A
∠BAC
∠CAB or ∠1.

Picture of Acute Angle
Degree is the unit used to represent an angle measure which is denoted by the symbol º. The measure of the angle around a circle is 360º.