29 CFR § 553.101 - “Volunteer” defined.
(a) An individual who performs hours of service for a public agency for civic, charitable, or humanitarian reasons, without promise, expectation or receipt of compensation for services rendered, is considered to be a volunteer during such hours. Individuals performing hours of service for such a public agency will be considered volunteers for the time so spent and not subject to sections 6, 7, and 11 of the FLSA when such hours of service are performed in accord with sections 3(e)(4) (A) and (B) of the FLSA and the guidelines in this subpart.
(b) Congress did not intend to discourage or impede volunteer activities undertaken for civic, charitable, or humanitarian purposes, but expressed its wish to prevent any manipulation or abuse of minimum wage or overtime requirements through coercion or undue pressure upon individuals to “volunteer” their services.
(c) Individuals shall be considered volunteers only where their services are offered freely and without pressure or coercion, direct or implied, from an employer.
(d) An individual shall not be considered a volunteer if the individual is otherwise employed by the same public agency to perform the same type of services as those for which the individual proposes to volunteer.