noun
convex angle
Geometric term for an angle less than 180 degrees; used in mathematics and technical contexts.
三角形の内角はすべて凸角である。
All interior angles of a triangle are convex angles.
凹角 (concave angle) is the opposite, an angle greater than 180 degrees.
Compound of 凸 (convex) and 角 (angle). The reading とっかく is the on'yomi of the characters.