Two morphisms in a category $C$ are parallel if they have the same source and target. Equivalently a pair of parallel morphisms in $C$ consists of an object$x$, and object $y$, and two morphisms $f, g: x \to y$.

$\array{
x
&
\underoverset
{\underset{g}{\longrightarrow}}
{\overset{f}{\longrightarrow}}
{}
&
y
}$

This may be extended to a family of any number of morphisms, but the morphisms are always compared pairwise to see if they are parallel. Degenerate cases: a family of one parallel morphism is simply a morphism; a family of zero parallel morphisms is simply a pair of objects.

Limits and colimits

The limit of a pair (or family) or morphisms is called their equalizer; the colimit is their coequalizer. (Of course, these do not always exist.)