FIFO is an acronym for First In, First Out. This expression describes the principle of a queue: what comes in first is handled first, what comes in next waits until the first is finished, etc. Thus it is analogous to the behaviour of persons "standing in a line" (preferred in American English) or "queue" (preferred in British English), where the persons leave the queue in the order they arrive.
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. Queuing theory encompasses the more general concept of queue, as well as interactions between strict-FIFO queues.
The expression FIFO can be used in different context:
In computer science this term refers to the way data stored in a queue is processed. Each item in the queue is stored in a queue (simpliciter) data structure. The first data to be added to the queue will be the first data to be removed, then processing proceeds sequentially in the same order. This is typical behavior for a queue, but see also the LIFO and stack algorithms.
In computing environments that support the pipes and filters model for interprocess communication, a FIFO is another name for a named pipe.
In electronics a FIFO is a semiconductor memory in which the first data to be written is always the first data to be read. The function is available as an integrated circuit that includes address counters, control logic and SRAM. A FIFO with a clock input is called a synchronous FIFO; otherwise it is asynchronous. The device typically has outputs called flags that indicate when it is empty or full.
In accounting, FIFO is a common method for approximating the value of inventory. It is appropriate where there are many different batches of similar products. The method presumes that the next item to be shipped will be the oldest of that type in the warehouse. In practice, this reflects the underlying commercial substance. See also LIFO in this context.
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