nLab maximal sieve

A sieve over an object $c$ of category $C$ is the maximal sieve over $c$ if it contains all morphisms with target $c$. In other words, it agrees with the class of all objects of the slice category $C/c$. One of the axioms of Grothendieck topologies says that any maximal sieve (that is the maximal sieve for any object in $C$) is a covering sieve.

A maximal sieve is any sieve generated by an identity morphism in $C$ (recall that a sieve generated by a family of morphisms $\{g_i:c_i\to c\}_{i\in I}$ is the class of all morphisms of the form $g_i\circ h$ where $h:e\to c_i$ is a morphism in $C$). If a sieve over $c$ is considered as a subpresheaf of the representable presheaf $Hom(-,c)$, then a sieve is maximal iff it is $Hom(-c)$.