Cuemath
Cuemath: Polyhedron
Introduces the concept of polyhedrons by learning the definition, types, and where they are found. Check-out the solved examples for better understanding and try your hand at solving a few interactive questions at the end of the page.
Wolfram Research
Wolfram Math World: Convex Polyhedron
This site defines a convex polyhedron in terms of vectors and matrices. A table of examples, as well as an illustration, are given. This site has numerous links to related Math World sites.
Science and Mathematics Initiative for Learning Enhancement (SMILE)
Smile: Polyhedrons
This in-depth paper examines the idea of convexity. Euclid's Elements, the first geometry textbook, assumed convexity without mentioning the term.