FIFO

FIFO is an acronym for First In, First Out, which is an abstraction related to ways of organizing and manipulation of data relative to time and prioritization. This expression describes the principle of a queue processing technique or servicing conflicting demands by ordering process by first-come, first-served (FCFS) behaviour: where the persons leave the queue in the order they arrive, or waiting one's turn at a traffic control signal.

FCFS is also the jargon term for the FIFO operating system scheduling algorithm, which gives every process CPU time in the order they come. In the broader sense, the abstraction LIFO, or Last-In-First-Out is the opposite of the abstraction FIFO organization. The difference perhaps is clearest with considering the less commonly used synonym of LIFO, FILO (meaning First-In-Last-Out). In essence, both are specific cases of a more generalized list (which could be accessed anywhere). The difference is not in the list (data), but in the rules for accessing the content. One sub-type adds to one end, and takes off from the other, its opposite takes and puts things only on one end.

A slang variation on an ad-hoc approach to removing items from the queue has been coined as OFFO, which stands for On-Fire-First-Out. A priority queue is a variation on the queue which does not qualify for the name FIFO, because it is not accurately descriptive of that data structure's behavior. Queueing theory encompasses the more general concept of queue, as well as interactions between strict-FIFO queues.