This entry is about the notion of site in 2-category theory. For the notion “bisite” of a 1-categorical site equipped with two coverages see instead separated presheaf.
2-natural transformation?
The notion of 2-site is the generalization of the notion of site to the higher category theory of 2-categories (bicategories).
Over a 2-site one has a 2-topos of 2-sheaves.
A coverage on a 2-category $C$ consists of, for each object $U\in C$, a collection of families $(f_i: U_i\to U)_i$ of morphisms with codomain $U$, called covering families, such that
This is the 2-categorical analogue of the 1-categorical notion of coverage introduced in the Elephant.
A 2-category equipped with a coverage is called a 2-site.
If $C$ is a regular 2-category, then the collection of all singleton families $(f:V\to U)$, where $f$ is eso, forms a coverage called the regular coverage.
Likewise, if $C$ is a coherent 2-category, the collection of all finite jointly-eso families forms a coverage called the coherent coverage.
On $Cat$, the canonical coverage consists of all families that are jointly essentially surjective on objects.
A pre-Grothendieck coverage on a 2-category is a coverage satisfying the following additional conditions:
If $f:V\to U$ is an equivalence, then the one-element family $(f:V\to U)$ is a covering family.
If $(f_i:U_i\to U)_{i\in I}$ is a covering family and for each $i$, so is $(h_{i j}:U_{i j} \to U_i)_{j\in J_i}$, then $(f_i h_{i j}:U_{i j}\to U)_{i\in I, j\in U_i}$ is also a covering family.
This is the 2-categorical version of a Grothendieck pretopology (minus the common condition of having actual pullbacks).
Now, a sieve on an object $U\in C$ is defined to be a functor $R:C^{op}\to Cat$ with a transformation $R\to C(-,U)$ which is objectwise fully faithful (equivalently, it is a fully faithful morphism in $[C^{op},Cat]$). Equivalently, it may be defined as a subcategory of the slice 2-category $C/U$ which is closed under precomposition with all morphisms of $C$.
Every family $(f_i\colon U_i\to U)_i$ generates a sieve by defining $R(V)$ to be the full subcategory of $C(V,U)$ on those $g:V\to U$ such that $g \cong f_i h$ for some $i$ and some $h:V\to U_i$. The following observation is due to StreetCBS.
A 2-presheaf $X:C^{op}\to Cat$ is a 2-sheaf for a covering family $(f_i:U_i\to U)_i$ if and only if
is an equivalence, where $R$ is the sieve on $U$ generated by $(f_i:U_i\to U)_i$.
Therefore, just as in the 1-categorical case, it is natural to restrict attention to covering sieves. We define a Grothendieck coverage on a 2-category $C$ to consist of, for each object $U$, a collection of sieves on $U$ called covering sieves, such that
If $R$ is a covering sieve on $U$ and $g:V\to U$ is any morphism, then $g^*(R)$ is a covering sieve on $V$.
For each $U$ the sieve $M_U$ consisting of all morphisms into $U$ (the sieve generated by the singleton family $(1_U)$) is a covering sieve.
If $R$ is a covering sieve on $U$ and $S$ is an arbitrary sieve on $U$ such that for each $f:V\to U$ in $R$, $f^*(S)$ is a covering sieve on $V$, then $S$ is also a covering sieve on $U$.
Here if $R$ is a sieve on $U$ and $g:V\to U$ is a morphism, $g^*(R)$ denotes the sieve on $V$ consisting of all morphisms $h$ into $V$ such that $g h$ factors, up to isomorphism, through some morphism in $R$.
As in the 1-categorical case, one can then show that every coverage generates a unique Grothendieck coverage having the same 2-sheaves.
The 2-category of 2-sheaves on a 2-site is a Grothendieck 2-topos.
If $C$ is a 1-category regarded as a 2-category with only identity 2-morphisms, then a coverage (pretopology, topology) on $C$ reduces to the usual notion of coverage, Grothendieck pretopology, or Grothendieck topology.
Strict 2-sites were considered in
Bicategorical 2-sites in
See also StreetCBS.
More discussion is in