Source
What this is
The source of a directed edge e : u → v is u — the vertex or object the edge departs from.
In directed graph theory: source = tail of the edge. The edge goes from tail to head. In category theory: source of a morphism f : A → B is A. Synonym for domain. In Petri nets: the source places of a transition t are its preset •t — the places that supply tokens.
Source is the same concept as Domain of a function, applied to the directed-graph setting where edges have explicit endpoints rather than being defined by a rule.