The time it takes for a radioactive isotope to halve its rate of radiation output (or, equivalently, the time it takes for half its mass to have decayed)
*dons sad scientist hat* Actually much more widely used to describe processes where the amount of a material in a system decays exponentially. Another example than that above is the measure of how long a given substance remains in a patient before they eliminate it (I know this because it's about all I understand from the articles I get from the ClinSchool? Library). *removes hat* *looks at the above* Did I write that? Help! Give me coffee! --MJ
Also a highly popular computer game, wherein a scientist working in a hard-to-navigate complex accidentally opens a portal to another world that lets in a number of aliens.