Definition 41.9.1. Flatness of modules and rings.
A module $N$ over a ring $A$ is said to be flat if the functor $M \mapsto M \otimes _ A N$ is exact.
If this functor is also faithful, we say that $N$ is faithfully flat over $A$.
A morphism of rings $f : A \to B$ is said to be flat (resp. faithfully flat) if the functor $M \mapsto M \otimes _ A B$ is exact (resp. faithful and exact).