What is a life line in any trivia quiz?

The term "lifeline" as used in trivia games originated in the TV show "Who Wants to Be a Millionaire?" It refers to permissible cheats or aids that a contestant can utilize if s/he is stumped. In the "Millionaire" game in the USA the lifelines originally consisted of 50/50 (whereby the computer reduces four answer choices to two), Ask-the-Audience (whereby the audience is polled), and Phone-a-Friend (whereby the contestant asks a pre-arranged acquaintance for the answer).
https://en.wikipedia.org/wiki/Who_Wants_to_Be_a_Millionaire%3F#Lifelines

Many game shows, pub quizzes, and other trivia games have adopted the lifeline concept, often dubbing it with their own quirky moniker such as a "shout-out" in the "Cash Cab" TV show.
dsc.discovery.com/tv-shows/cash-cab webpage no longer exists

The "Millionaire" show borrowed term "lifeline" from the life-saving and boating professions. A lifeline may be "a line or rope for saving life, as one attached to a lifeboat" or "any of various lines running above the decks, spars, etc., of a ship or boat to give sailors something to grasp when there is danger of falling or being washed away".

http://dictionary.reference.com/browse/lifeline