Rubber stamp

What are rubber stamps? I did a Google search for "rubber stamp definition" and came up with some results:

The American Heritage Dictionary said: A person or body that gives perfunctory approval or endorsement of a policy without assessing its merit. A perfunctory approval or endorsement.

BARRON'S Marketing Dictionary said: Term used to refer to individuals who, with little thought, approve everything that crosses their desks (e.g., rubber-stamp executives or politicians).

The Houghton Mifflin Company said: A person or organization that automatically approves or endorses a policy without assessing its merit; also, such an approval or endorsement. For example, The nominating committee is merely a rubber stamp; they approve anyone the chairman names, or The dean gave his rubber stamp to the recommendations of the tenure committee. This metaphoric term alludes to the rubber printing device used to imprint the same words over and over. [Early 1900s]

Lexicus said: Routine authorization of an action without questions.

YourDictionary.com said: a person, bureau, legislature, etc. that approves or endorses something in a routine manner, without thought

Merriam-Webster said: to approve, endorse, or dispose of as a matter of routine or at the command of another