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Opposite of concave . A geometric shape is convex if all its edges point outwards. That is no part of it curves or faces inwards. If a shape is convex, a line segment drawn between any two points on the shape will always lie inside the shape. The figure above is a convex pentagon. No part of it is caved in and any line segment, such as the red one shown, linking two points on it always lies inside the shape. See also Concave definition.
A polyhedron is any three-dimensional figure with flat surfaces that are polygons. Specifically, any geometric shape existing in three-dimensions and having flat faces, each existing in two-dimensions, which intersect at straight, linear edges. The edges themselves intersect at points called vertices.