1a rope or line used for life-saving, typically one thrown to rescue someone in difficulties in water or one used by sailors to secure themselves to a boat:he rigged a lifeline fore and aft and clipped the safety line on the girl’s life jacket to it
a line used by a diver for sending signals to the surface.
2a thing on which someone or something depends or which provides a means of escape from a difficult situation:the telephone has always been a lifeline for Gabby and me
3(in palmistry) a line on the palm of a person’s hand, regarded as indicating how long they will live.
throw a lifeline to
provide (someone) with a means of escaping from a difficult situation:women who feel at risk of breaking down alone have been thrown a lifeline by a motoring organization
lifeline in other Oxford dictionaries
Definition of lifeline in the US English dictionary