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A plane figure with three straight sides and three angles.
A polygon.
A rectangle is a plane figure with four straight sides and four right angles, esp. one with unequal adjacent sides. A square is a plane figure with four equal straight sides and four right angles. A rhombus is a parallelogram with opposite equal acute angles, opposite equal obtuse angles, and four equal sides.
A polygon is a plane figure with at least three straight sides and angles, so a triangle qualifies.
A plane figure with at least three straight sides and angles, and typically five or more.