Let $M$ be a smooth manifold. Then what is meant by saying that a map $c : C^{\infty} (M) \times C^{\infty} (M) \longrightarrow C^{\infty} (M)$ is a bidifferential operator?
Such terminology has been used by Etingof in his lecture notes on quantum groups without explicitly mentioning what it is actually. Does anyone have any idea?
Thanks for your time.