means The act of traveling across water in a boat propelled by the wind in its sails, or, more loosely, the skill and sport of doing so.

from From Old English 'segl,' the noun for a sail — that broad sheet of cloth hung to catch the wind — which gave the verb 'seglian,' to travel by sail. It belongs to a wide Germanic family (Old Norse 'segl,' German 'Segel'), all naming the same wind-hungry canvas. The word predates engines by centuries, so to 'sail' once meant, quite simply, the only way a ship moved at all.