The d20 is a platonic solid, the icosahedron. 20-sided dice are the standard dice used in Dungeons and Dragons as well as many other RPGs, and is often used as a symbol of roleplaying games in general. After the d6, the d20 is the second most popular die sold: though D&D requires only one d20 to be rolled at any given time, many gamers prefer to have a selection of 20 sided dice at hand to roll for different characters, in different situations, or when the luck of one die seems to be giving out.
The oldest d20s recorded is from Egypt and is dated to 300 BC. An ancient Roman d20 dated to 100 AD sold at auction for nearly $18,000 in 2003. Despite the fact that the ancient Greeks had a documented understanding of the platonic solids, including the icosahedron, the d20 was actually patented in 1950 by the Tokyo Shibaura Electric Company. When D&D was first released with a dice set, the d20 was numbered 0-9 twice.