n-category = (n,n)-category
n-groupoid = (n,0)-category
A bicategory is a particular algebraic notion of weak 2-category (in fact, the earliest to be formulated, and still the one in most common use). The idea is that a bicategory is a category weakly enriched over Cat: the hom-objects of a bicategory are hom-categories, but the associativity and unity laws of enriched categories hold only up to coherent isomorphism.
A bicategory consists of
If there is exactly one -cell, say , then the definition is exactly the same as a monoidal structure on the category . This is one of the motivating examples behind the delooping hypothesis and the general notion of k-tuply monoidal n-category.
Here we spell out the above definition in full detail. Compare to the detailed definition of strict -category, which is written in the same style but is simpler.
A bicategory consists of
It is quite possible that there are errors or omissions in this list, although they should be easy to correct. The point is not that one would want to write out the definition in such elementary terms (although apparently I just did anyway) but rather that one can.
Any strict 2-category is a bicategory in which the unitors and associator are identities. This includes Cat, MonCat?, the algebras for any strict 2-monad, and so on, at least as classically conceived.
Categories, anafunctors, and natural transformations, which is a more appropriate definition of Cat in the absence of the axiom of choice, form a bicategory that is not a strict 2-category. Indeed, without the axiom of choice, the proper notion of bicategory is anabicategory.
The fundamental 2-groupoid? of a space is a bicategory which is not necessarily strict (although it can be made strict fairly easily when the space is Hausdorff by quotienting by thin homotopy, see path groupoid and fundamental infinity-groupoid). When the space is a CW-complex, there are easier and more computationally amenable equivalent strict 2-categories, such as that arising from the fundamental crossed complex.
One way to state the coherence theorem for bicategories is that every bicategory is equivalent to a strict -category. This “strictification” is not obtained naively by forcing composition to be associative, but (at least in one construction) by freely adding new composites which are strictly associative. Another way to state the coherence theorem is that every formal diagram of the constraints (associators and unitors) commutes.
The proof of the coherence theorem is basically the same as the proof of the coherence theorem for monoidal categories. An abstract approach can be found in Power’s paper “A general coherence result.”
Classically, “2-category” meant strict 2-category, with “bicategory” used for the weak notion. This led to the more general use of the prefix “2-” for strict (that is, strictly Cat-enriched) notions and “bi-” for weak ones. For example, classically a “2-adjunction” means a Cat-enriched adjunction, consisting of two strict 2-functors and a strictly Cat-natural isomorphism of categories , while a “biadjunction” means the weak version, consisting of two weak 2-functors and a pseudo natural equivalence . Similarly for “2-equivalence” and “biequivalence,” and “2-limit” and “bilimit.”
We often use “2-category” to mean a strict or weak 2-category without prejudice, although we do still use “bicategory” to refer to the particular classical algebraic notion of weak 2-category. We try to avoid the more general use of “bi-” meaning “weak,” however. For one thing, is it confusing; a “biproduct” could mean a weak 2-limit, but it could also mean an object which is both a product and a coproduct (which happens quite frequently in additive categories).
Moreover, in most cases the prefix is unnecessary, since once we know we are working in a bicategory, there is usually no point in considering strict notions at all. Fully weak limits are really the only sensible ones to ask for in a bicategory, and likewise for fully weak adjunctions and equivalences. Even in a strict 2-category, while we might need to say “strict” sometimes to be clear, we don't need to say “-”, since we know that we are not working in a mere category. (Max Kelly pushed this point.)
When we do have a strict 2-category, however, other strict notions can be quite technically useful, even if our ultimate interest is in the weak ones. This is somewhat analogous to the use of strict structures to model weak ones in homotopy theory; see here and here for good introductions to this sort of thing.
Discussion about the use of the term “weak enrichment” above is at weak enrichment.
See the references at 2-category.