Is it possible to define a ring as a category? Is it possible to define a ring as a category? For example, a group can be defined as a category with just one object and all morphisms being isomorphisms.
 A: A monoid is a category with exactly one object. A homomorphism of monoids is just a functor of the corresponding categories.
A ring is a linear category with exactly one object. A homomorphism of rings is just a linear functor between the corresponding linear categories.
More generally, if $R$ is a commutative ring, then an $R$-algebra is an $R$-linear category with exactly one object, and $R$-algebra homomorphisms correspond to $R$-linear functors.
This offers a compact definition of modules: A left module over $R$ is just a linear functor $R \to \mathsf{Ab}$. Actually this has lead to the following more general notion ( which has found applications in category theory and algebraic topology): If $C$ is any linear category, then a $C$-module is a linear functor $C \to \mathsf{Ab}$. Homomorphisms of $C$-modules are natural transformations. Thus, we optain a category $C$-modules.
Besides, it shows that the category of monoids or rings is actually a bicategory: If $f,g : M \to N$ are homomorphisms of monoids (or rings), then a $2$-morphism $f \to g$ is an element $n \in N$ such that $n f(m) = g(m) n$ for all $m \in M$. 
