Definition: The wind experienced by a moving object (e.g. a boat).
If you put your hand out of a car window on a windless day you still feel some wind. This is as a result of the car moving. The direction of that wind is directly opposite the direction of the moving object. This wind also exists on sailboats as they move forward. But the sailor also feels the real wind. What the sailor feels is a combination of both.
This is called apparent wind. If the wind is strong then the apparent wind is influenced more by the actual wind. Conversely, the faster a boat travels the more of an influence boat speed has on the apparent wind.