A short, fast sprint, used to develop your wind (breath), speed, and endurance. Usually done as a series, running, resting, and running again.
To do a wind sprint correctly, start running as fast as you can, and stop when you're winded (hence the name). They will be the most effective if they are done alternating with less exhausting exercises.
This term is sometimes applied to other sports besides ones that involve running. Any short burst of activity that winds you (usually in an aerobic exercise) can be called a wind sprint.