Morita-equivalent modules

Definition with symbols
Two modules $$M_1$$ and $$M_2$$ over a commutative unital ring are said to be Morita-equivalent if there are projective modules $$P_1$$ and $$P_2$$ such that:

$$M_1 \oplus P_1 \cong M_2 \oplus P_2$$

Modules, modulo the relation of Morita equivalence, give rise to the Morita category.