What does this symbol mean?

[Tosh5457]

This one

in this context

I never understood what it means and I can't find its meaning...

 

[D H]

It means union. In this particular case, it is the union, for all subsets G of the set T, of the sets of ordered pairs of the elements constructible from the set G.

In other words, it is the set of all mappings (of functions) of T into itself.