From The Lexicon of Sustainability:

CAFO: Concentrated Animal Feedlot Operations

An animal feeding operation that confines animals for more than 45 days during a growing season in an area that does not produce vegetation. CAFOs substitute structures and equipment (with regards to feeding, temperature controls and manure management) for land and labor. The United States EPA has created three categories of CAFOs in terms of their capacity: large, medium and small. The corresponding animal unit for each of these categories depends on the species of animal. 

(Learn more about CAFOs at the Lexicon of Sustainability)