The Collaborative International Dictionary of English v.0.48:

Lifeboat \Life"boat`\ (l[imac]f"b[=o]t`), n.
   A strong, buoyant boat especially designed for saving the
   lives of shipwrecked people.
   [1913 Webster]

WordNet (r) 3.0 (2006):

lifeboat
    n 1: a strong sea boat designed to rescue people from a sinking
         ship