The acorn is the fruit of the oak tree (genera Quercus, Lithocarpus and Cyclobalanopsis, in the family Fagaceae). It is a nut, containing a single seed (rarely two seeds), enclosed in a tough, leathery shell, and borne in a cup-shaped cupule. Acorns vary from 1 – 6 cm long and 0.8 – 4 cm broad. Acorns take between about 6 or 24 months (depending on the species) to mature; see List of Quercus species for details of oak classification, in which acorn morphology and phenology are important factors.

ACORN, the Association of Community Organizations for Reform Now, a community organization of low- and moderate-income families that addresses housing, schools, neighborhood safety, health care, job conditions, and other social issues that affect its members. With a membership of over 350,000, ACORN is organized into more than 850 neighborhood chapters in over 100 cities across the United States, as well as in Argentina, Canada, Mexico, and Peru. The organization was born out of the American Civil Rights Movement. ACORN was founded by Wade Rathke, a community and labor organizer, in 1970. Maude Hurd has been National President of ACORN since 1990.