Organic is a system of farming, food processing, and a label. In the U.S., organic is regulated by the National Organic Program (NOP) of the Department of Agriculture’s (USDA) Agricultural Marketing Service (AMS), in accordance with the Organic Foods Production Act of 1990 (OFPA). The following definition, excerpted from the law, provides the legal basis for regulating organic farming and food production in the U.S.
Organic is defined as “a production system that is managed…to respond to site-specific conditions by integrating cultural, biological, and mechanical practices that foster cycling of resources, promote ecological balance, and conserve biodiversity. Synthetic fertilizers, sewage sludge, irradiation, and genetic engineering may not be used.” (7 CFS § 205.2 Subpart A — definitions.)
National Organic Standards Board (NOSB) 
This more descriptive definition, adopted by the NOSB prior to the passage of the final organic regulations, explains in a more nuanced way what organic is and isn’t in practice.
"Organic agriculture is an ecological production management system that promotes and enhances biodiversity, biological cycles and soil biological activity. It is based on minimal use of off-farm inputs and on management practices that restore, maintain and enhance ecological harmony.
‘Organic’ is a labeling term that denotes products produced under the authority of the Organic Foods Production Act. The principal guidelines for organic production are to use materials and practices that enhance the ecological balance of natural systems and that integrate the parts of the farming system into an ecological whole.
Organic agriculture practices cannot ensure that products are completely free of residues; however, methods are used to minimize pollution from air, soil and water.
Organic food handlers, processors and retailers adhere to standards that maintain the integrity of organic agricultural products. The primary goal of organic agriculture is to optimize the health and productivity of interdependent communities of soil life, plants, animals and people" (Adopted April 1995).
International Federation of Organic Food Movements (IFOAM) 
This internationally recognized definition has been adopted by a federation of over 750 organic agriculture movement organizations in 108 countries.
“Organic agriculture is a production system that sustains the health of soils, ecosystems and people. It relies on ecological processes, biodiversity and cycles adapted to local conditions rather than the use of inputs with adverse effects. Organic agriculture combines tradition, innovation and science to benefit the share environment and promote fair relationships and a good quality of life for all involved.” (Adopted March 2008)
 OFPA is the law that governs the growing, handling, processing and labeling of organic food.
 The NOSB is a voluntary Federal Advisory Committee Act (FACA) appointed by the U.S. Secretary of Agriculture to make recommendations to the National Organic Program (NOP) and the Secretary on matters related to OFPA implementation. Comprised of 15 members from the organic stakeholder communities across the U.S., the NOSB possess sole authority to add or remove materials from National List of Allowed and Prohibited Substances, based upon public and industry input received in writing and at its bi-annual meeting.
 IFOAM consists of advocates and practitioners of organic agriculture from around the world. It was formed in 1972 to democratically draft organic processing and handling standards that revolve around the principles of health, ecology, fairness, and care.