The most common wind direction in a given area.  May be determined by observation over time or my be deduced by noting patterns in vegetation and surface features.  For example, trees may lean in a common direction, or have branches swept to one side, or simply many more branches on one side than the other.  The prevailing would could be presumed to be coming from the side leaned or swept away from.  Likewise sand dunes are generally steeper on the downwind side than on the upwind side.