Full Idea
'dom R' indicates the 'domain' of a relation, that is, the set of all objects that are members of ordered pairs and that have that relation.
Gist of Idea
'dom R' indicates the 'domain' of objects having a relation
Source
Herbert B. Enderton (A Mathematical Introduction to Logic (2nd) [2001], Ch.0)
Book Reference
Enderton,Herbert B.: 'A Mathematical Introduction to Logic' [Academic Press 2001], p.4