26 CFR § 1.509(a)-1 - Definition of private foundation.
In general. Section 509(a) defines the term private foundation to mean any domestic or foreign organization described in section 501(c)(3) other than an organization described in section 509(a) (1), (2), (3), or (4). Organizations which fall into the categories excluded from the definition of private foundation are generally those which either have broad public support or actively function in a supporting relationship to such organizations. Organizations which test for public safety are also excluded.