interior angle

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. Any of the four angles formed between two straight lines intersected by a third straight line.
2. n. The angle formed inside a polygon by two adjacent sides.

Wiktionary

1. n. geometry The inner angle between two sides of a polygon.

GNU Webster's 1913

1. adj. (Geom.) an angle formed between two sides, within any rectilinear figure, as a polygon, or between two parallel lines by these lines and another intersecting them; -- called also internal angle.

WordNet 3.0

1. n. the angle inside two adjacent sides of a polygon