A wind blowing in the opposite direction to one's intended course or travel.
From counter- (opposite) + wind (moving air). A nautical and aviation term that emerged from practical needs of sailors and pilots to describe headwinds and opposing air currents.
Sailors use this constantly—it's when the wind is actively fighting against you, which is why sailing into a counterwind requires zigzagging back and forth (called tacking) rather than going straight.
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