= interior angle n. at interior, adj. 1a.

the interior angle formed by any two things which intersect or join at a point (esp. adjacent sides of a triangle).

a re-entrant angle (see re-entrant, adj. 1b).

Situated more within, or (usually, simply) within, something; belonging to or connected with the inside; = inner, adj. 1a, internal, adj. 1.

In other contexts: a re-entrant angle or angular shape; (Geomorphology) a prominent angular indentation in a landform, such as an inlet between two…