A paracategory is a category where composition is only partially defined.
The definition is “unbiased” in that it comes with basic (partial) $n$-ary composition operations for all $n$, and unlike in the total case these cannot always be reduced to binary and nullary operations. A paracategory in which all the composites are generated from binary and nullary ones is sometimes called a precategory.
A paracategory is a quiver $C_1 \rightrightarrows C_0$ together with the following structure. We write $C_n$ for the iterated pullback $C_1 \times_{C_0} \dots\times_{C_0} C_1$, the set of length-$n$ strings of composable arrows.
Partial functions $\circ_n \colon C_n ⇀ C_1$, over $C_0\times C_0$
$\circ_0 \colon C_0 \to C_1$ is total, so all identity arrows exist.
$\circ_1\colon C_1 \to C_1$ is the identity.
If $\circ_n \vec{y}$ is defined, then $\circ_{m+1+k}(\vec{x},\circ_n \vec{y},\vec{z}) = \circ_{m+n+k}(\vec{x},\vec{y},\vec{z})$ (the equality being Kleene equality).
A functor between paracategories is a quiver morphism $f\colon C\to D$ such that if $\circ_n \vec{x}$ is defined, then so is $\circ_n \vec{f(x)}$ and it equals $f(\circ_n \vec{x})$. A Kleene functor is a functor such that if $\circ_n \vec{f(x)}$ is defined, then so is $\circ_n \vec{x}$; this is equivalently a quiver morphism such that $\circ_n \vec{f(x)}=f(\circ_n \vec{x})$ is a Kleene equality.
If $D$ is any category and $C$ is a subclass of arrows in $D$ containing all the identities, then $C$ becomes a paracategory whose objects are the objects of $D$, whose arrows are the arrows in $C$, and where the composite of a string of arrows is defined iff the composite of that string in $D$ happens to lie in $C$.
extranatural transformations and dinatural transformations form paracategories, since they are not always composable. This is exploited in the definition of extraordinary 2-multicategory?. This example can also be regarded as a case of the previous example, where the ambient category $D$ consists of “unnatural transformations.”
In fact, if $Cat_P$ denotes the category of categories equipped with a subclass of arrows containing the identities, then the functor $Cat_P \to ParCat$ defined above is actually a coreflection, i.e. it has a fully faithful left adjoint. In particular, any paracategory is isomorphic to one obtained from a class of arrows in some category, and moreover in a universal way.
Paracategories, and more general “partial algebras,” can be considered as a special case of generalized multicategories; see the papers of Hermida.
The definition is due to Peter Freyd in apparently unpublished work. It has been studied further in the papers:
Hermida and Mateus, “Paracategories I: Internal Paracategories and Saturated Partial Algebras”
Hermida and Mateus, “Paracategories II: Adjunctions, fibrations and examples from probabilistic automata theory”