lifeline

noun

1. a rope or line for saving life, as one thrown to a person in the water
2. any rope strung along or above a vessel's weather deck to provide a handhold as in rough weather
3. the rope by means of which a diver is raised and lowered, used by the diver for signaling
4. Palmistry a line in the palm of the hand, curving about the base of the thumb, that reveals facts about a person's life
5. a commercial, esp. maritime, route of great importance
6. a route that is the only one over which supplies can be transported to a certain place