In which reference can I find a definition for the equivalence of DFAs?
You can find a definition in Automata Theory, Languages and Computation by Hopcroft, Motwani, Ullman, in the fourth chapter.

Here's a link : HMU, the section begins at page 171 of the pdf

Two DFA are said to be equivalent if they accept the same language, and (an equivalent proposition) if they have the same minimal automaton (which is unique).