What is the meaning of "dom(R)=\left \{ u:\exists v(u,v)\in R\right \}"?

Question

What is the meaning of "
`dom(R)=\left \{ u:\exists v(u,v)\in R\right \}`"?

Answer 1

It means that the domain of the relation
`R` is the set of such elements
`u` for which there exists such an element
`v` that
`u` and
`v` are related.