Proof-of-payment (POP) or proof-of-fare (POF) is an honor-based fare collection system used on many public transportation systems. Instead of checking each passenger as they enter a fare control zone, passengers are required to carry a paper ticket, transit pass, transit smartcard - or open payment methods such as contactless credit or debit cards (if applicable) - after swiping or tapping on smart card readers, to prove that they have paid the valid fare. Fares are enforced via random spot-checks by inspectors such as conductors or enforcement officers, to ensure that passengers have paid their fares and are not committing fare evasion. On many systems, a passenger can purchase a single-use ticket or multi-use pass at any time in advance, but must insert the ticket or pass into a validation machine immediately before use. Validation machines in stations or on board vehicles time stamp the ticket. The ticket is then valid for some period of time after the stamped time.

A fare is the fee paid by a passenger for use of a public transport system: rail, bus, taxi, etc. In the case of air transport, the term airfare is often used. Fare structure is the system set up to determine how much is to be paid by various passengers using a transit vehicle at any given time. A linked trip is a trip from the origin to the destination on the transit system. Even if a passenger must make several transfers during a journey, the trip is counted as one linked trip on the system.