Definition of "FIFO (First In, First Out)"

Method of accounting for the purchase and sale of securities for tax purposes whereby the first security purchased is assumed to be the first security sold. For instance, under FIFO, or first in, first out accounting, an investor who purchased 100 shares of XYZ in January and another 100 shares of XYZ in March, and then sold 100 shares of XYZ in November, would have sold the first 100 shares bought in January. In contrast, the LIFO method, or last in, first out would allocate the shares bought in March as the shares sold.