A lifeline is a form of aid which is given to contestants in all worldwide formats of the show. Most of the time, each lifeline can be used by the contestant only once during their game to help them on a particular question. However, contestants can usually use more than one lifeline during the same question in any order they want. Each lifeline has a different effect, and lots of contestants use them strategically (for example, not using a lifeline during the question after a safety net has been achieved, as it usually is a free shot).
Most formats use three lifelines available to the contestant (50:50, Phone-A-Friend and Ask the Audience being the most popular ones), however, some versions offer more lifelines (sometimes added in after a certain point) and some may even have less than three of them in total.