symmetric monoidal (∞,1)-category of spectra
A local ring is a ring (with unit, usually also assumed commutative) such that: * $0 \ne 1$; and * whenever $a + b = 1$, $a$ or $b$ is invertible.
Here are a few equivalent ways to phrase the combined condition: * Whenever a (finite) sum equals $1$, at least one of the summands is invertible. * Whenever a sum is invertible, at least one of the summands is invertible. * Whenever a sum of products is invertible, for at least one of the summands, all of its multiplicands are invertible. * The non-invertible elements form an ideal. (Unlike the previous clauses, this requires excluded middle to be equivalent.)
The ideal of non-invertible elements is in fact a maximal ideal, so the quotient ring is a field. (This quotient can also be taken constructively, where one mods out by an anti-ideal.)
In algebraic geometry or synthetic differential geometry and commutative algebra, the most commonly used definition of a local commutative ring is a commutative ring $R$ with a unique maximal ideal. Hence the Spec of such an $R$ has a unique closed point. Intuitively it can be thought of as some kind of “infinitesimal neighborhood” of a closed point.
The topos theory formulation of this is a local topos.
An important example of a local ring in algebraic geometry is $R = k[\epsilon]/\epsilon^2$. This ring is known as the ring of dual numbers. Intuitively, we can think of its spectrum as consisting of a closed point and a tangent vector. Indeed this is justified, as morphisms from $\operatorname{Spec} R$ to a scheme $X$ correspond exactly to pairs $(x,v)$, where $x \in X$ and $v$ is a (Zariski) tangent vector at $x$.
Local rings are also important in deformation theory. One might define an infinitesimal deformation of a scheme $X_0$ to be a deformation of $X_0$ over $\operatorname{Spec} R$ where $R$ is a local ring.
Local rings are often more useful than fields when doing mathematics internally. For one thing, the definition make sense in any coherent category. But unlike the definition of discrete field, which is also coherent, it is satisfied by rings such as the ring of (located Dedekind) real numbers. Rather than mod out by the ideal of non-invertible elements, you take care to use only properties that are invariant under multiplication by an invertible element.
In constructive mathematics, one could do the same thing, but it's more common to use the notion of Heyting field. This is closely related, however; the quotients of local rings are precisely the Heyting fields (which are themselves local rings). In fact, one can define an apartness relation (like that on a Heyting field) in any local ring: $x \# y$ iff $x - y$ is invertible. Then the local ring is a Heyting field if and only if this apartness relation is tight.