The terminal side of an angle θ, in standard position, is the ray that rotates from the positive x-axis to define the angle.
In more detail:
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Standard Position: An angle is in standard position when its vertex is at the origin of a coordinate plane, and its initial side (the starting point of the rotation) lies along the positive x-axis.
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Initial Side: The initial side is the fixed ray on the positive x-axis.
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Terminal Side: The terminal side is the ray that moves, or rotates, away from the initial side. The amount of rotation determines the measure of the angle. The terminal side can lie in any of the four quadrants, or along an axis.
Essentially, visualize a line segment anchored at the origin (the vertex). One end is fixed on the positive x-axis (the initial side). The other end of the line segment rotates. The position where that rotating end stops is the terminal side, and the angle θ is defined by the amount of rotation.
