the amount of time it takes for a sample of radioactive material to lose half of its mass due to spontanious radiation.
Also a game that revolutionized gaming. There is a lot of speculation of why this game became so popular. The true reason is that the main character is a nerd and nerds play computer games. For the first time the nerd was a superhero. He was a scientist, and the idea of one kickin alien ass rather than hiding behind test-tubes was revolutionary and unrealistic.