d8 dice are a platonic solid known as the octahedron. Eight-sided dice are most commonly used as part of any roleplaying game that makes use of the standard D&D dice set, including D&D, Pathfinder, Earthdawn, and Call of Cthulhu (where 8 sided dice are mostly used for sanity losses). In D&D the 8-sided dice are best known as the damage dice for the longsword.

The oldest d8 we've been able to track down dates to 1888 and was used as poker dice.