First in, first out. A queue; a data structure or hardware buffer from which items are taken out in the same order they were put in. A FIFO is useful for buffering a stream of data between a sender and receiver which are not synchronized; that is, the sender and receiver are not sending and receiving at exactly the same rate. If the rates differ by too much in one direction for too long, the FIFO becomes either full (blocking the sender) or empty (blocking the receiver).