Basic Guidelines For English Spellings
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1Any of various kinds of (thin) rope or line used for a particular purpose, (now) especially in sailing.
2Nautical
A rope, wire, etc., running along the length of a ship's deck and secured at both ends, to which a crew member's safety harness is clipped so that he or she can move about the deck safely in rough weather, conditions of poor visibility, etc.
Early 17th century. From Jack + line.