Definition
Straight angle
A straight angle measures exactly 180°, which is one half of a full turn.
What it means
Start along one ray and turn halfway around. The second ray continues in the opposite direction, so the two rays look like one straight line.
Remember it
Half-turn, opposite rays, 180°.
Example
A ray points east from point P. A second ray points west from P. Turning from the first ray to the second is 180°, so the angle at P is a straight angle.
Watch for
An angle of 179° or 181° is not straight; the measure must be exactly 180°.
Common mix-up
A straight angle looks like a line, but it records a half-turn around a vertex.
Exact meaning
A straight angle is formed by two rays with a common vertex that point in opposite directions. Its measure is exactly 180°.