Definition of FIFO

First-in, first-out. A method of accounting for inventory which assumes that goods are sold in the order in which they are purchased, i.e., the oldest items sold first. The other common inventory costing methods include LIFO (last-in, first-out), specific identification, and average cost. Contrast with Last-in, first-out (LIFO).

That's the definition of FIFO in Black's Law Dictionary 6th Edition. Courtesy of

Official tim editorial.