lifeline

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun An anchored line thrown as a support to someone falling or drowning.
• noun A line shot to a ship in distress.
• noun A line used to raise and lower deep-sea divers.
• noun A means or route by which necessary supplies are transported.
• noun One that is or is regarded as a source of salvation in a crisis.
• noun A diagonal line crossing the palm of the hand and believed to indicate the length and major events of one's life.

from The Century Dictionary.

• noun A line used by firemen to lower people from a burning building.
• noun Nautical: A rope stretched anywhere on a vessel for the safety of the men in bad weather or when they are manning yards: in the latter case it is stretched from the mast to the lift.
• noun One of several lines attached to a life-buoy or life-boat, to enable a person in the water to reach the boat or buoy more readily.

from the GNU version of the Collaborative International Dictionary of English.

• noun The anem given to one of the creases on the palm; its length is said by palmists to indicate how long one will live.
• noun A line or rope which raises or lowers a deep-sea diver.
• noun A line from a vessel that people in a body of water can cling to to save themselves from drowning.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A line to which a drowning or falling victim may cling to.
• noun A means or route for transporting indispensable supplies.
• noun A source of salvation in a crisis.
• noun nautical On the deck of a boat, a line to which one can attach oneself to stay aboard on rough seas

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun line that raises or lowers a deep-sea diver
• noun line thrown from a vessel that people can cling to in order to save themselves from drowning
• noun support that enables people to survive or to continue doing something (often by providing an essential connection)
• noun a crease on the palm; its length is said by palmists to indicate how long you will live
• noun a crease on the palm; its length is said by palmists to indicate how long you will live