Dear W'on:

Yes, at first I thought the "becomes" in the title was the definition you just posted.

But now I think the "becomes" means (based on your information about the play) must mean to move from one state to another. Mourning = Electra.

Who knows? Mebbe O'Neill wanted us to think of the "becomes" in several ways... I haven't read the play so I'm really being ridiculous hypothesizing here.