While playing a shooting game you kill an enemy without scoping.

A noscope is pretty self explanatory. It's killing someone without scoping in with a sniper. Noscopes are often tried in the CoD series, especially on MW2 for a game winning killcam. If you get a noscope across map, you're pro. If people think you have an aimbot because the noscope was so awesome, you're offically a legend.