A convex decagon is one in which no angle is a reflex angle.
Another definition is that given any two points inside the decagon, the straight line joining the two points must lie wholly inside the decagon.
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In 2-d, the naming of polygons is such that a 10 sided shape is a decagon. Even if it is a 5-pointed star shape, it is a decagon. A concave decagon (rather than convex) but still a decagon. In 3-d, a decahedron, possibly, with the term "sides" being used for "faces".
Decagon
The anwser is a Decagon.
Decagon
Its not a do-decagon its a dodecahedran!!