From the American Association of Museums' website:
American museums are infinitely diverse. The AAM Code of Ethics for Museums notes that their common denominator is making a "unique contribution to the public by collecting, preserving, and interpreting the things of this world."

The code also acknowledges the variety of sizes and types of museums: "Their numbers include both governmental and private museums of anthropology, art history and natural history, aquariums, arboreta, art centers, botanical gardens, children's museums, historic sites, nature centers, planetariums, science and technology centers, and zoos."

To participate in the AAM Accreditation Program, a museum must:

• Be a legally organized nonprofit institution or part of a nonprofit organization or government entity
• Be essentially educational in nature
• Have a formally stated and approved mission
• Use and interpret objects and/or a site for the public presentation of regularly scheduled programs and exhibits
• Have a formal and appropriate program of documentation, care, and use of collections and/or objects
• Carry out the above functions primarily at a physical facility/site
• Have been open to the public for at least two years
• Be open to the public at least 1,000 hours a year
• Have accessioned 80 percent of its permanent collection
• Have at least one paid professional staff with museum knowledge and experience
• Have a full-time director to whom authority is delegated for day-to-day operations
• Have the financial resources sufficient to operate effectively
• Demonstrate it meets the Characteristics of an Accreditable Museum

To learn more about the AAM Accreditation program, use this link:
www.aam-us.org/museumresources/accred/standards.cfm

For the International Council of Museums (ICOM), a museum is defined as:

• A non-profit making, permanent institution in the service of society and of its development, and open to the public, which acquires, conserves, researches, communicates and exhibits, for purposes of study, education and enjoyment, material evidence of people and their environment.

The federal government in the Museum and Library Services Act defined a museum as: "A public or private nonprofit agency or institution organized on a permanent basis for essentially educational or aesthetic purposes, which, utilizing a professional staff, owns or utilizes tangible objects, cares for them, and exhibits them to the public on a regular basis."

The Institute of Museum and Library Services (IMLS, www.imls.gov) uses the Museum and Library Services Act definition as the basis for eligibility to receive federal funds from IMLS:

• Be organized as a public or private nonprofit institution that exists on a permanent basis for essentially educational or aesthetic reasons
• Care for and own or use tangible objects, whether animate or inanimate, and exhibit these objects on a regular basis through facilities that it owns or operates
• Have at least one professional staff member or the full-time equivalent, whether paid or unpaid, whose primary responsibility is the acquisition, care, or exhibition to the public of objects owned or used by the museum
• Be open and providing museum services to the general public for at least 120 days a year