The Collaborative International Dictionary of English v.0.48:

lifeline \lifeline\ n.
   1. The anem given to one of the creases on the palm; its
      length is said by palmists to indicate how long one will
      live.

   Syn: line of life, life line.
        [WordNet 1.5]

   2. A line or rope which raises or lowers a deep-sea diver.
      [WordNet 1.5]

   3. A line from a vessel that people in a body of water can
      cling to to save themselves from drowning.
      [WordNet 1.5]

WordNet (r) 3.0 (2006):

lifeline
    n 1: a crease on the palm; its length is said by palmists to
         indicate how long you will live [syn: line of life, life
         line, lifeline]
    2: support that enables people to survive or to continue doing
       something (often by providing an essential connection); "the
       airlift provided a lifeline for Berlin"; "she offered me a
       lifeline in my time of grief"
    3: line that raises or lowers a deep-sea diver
    4: line thrown from a vessel that people can cling to in order
       to save themselves from drowning