In the comic industry, a one-shot is a piece that is created as a pilot comic or stand-alone story. Many artists publish one-shots to test a possible new story, or to express information that would not fit with the story arc.
Example Usage of One-shot
JordanRockswell: One shot. One kill. Opportunity only knocks once.
bobwyo: Hmmmmm, gotta try aboutrc's One-shot HDR. Might even share.
jennyduh: No, i didnt get one shot, i got two. Ain't i luckyy. =/