A nollie is a trick done in skateboarding to get air. You place your front foot on the nose, back foot halfway back on the board, and pop the nose down, while sliding your back foot towards the tail. You then level out with the back foot, and stomp down to prevent board slippage.
Named for Natas Koupas, the inventor of the nollie. Nollie is short for Natas ollie, although many people will tell you it means nose ollie.