Does a definition with a disjunction (i.e. “something is an X if it fulfills either A or B”) lose its defining power? It somehow seems to make the definition weaker, though not by way of any weakness in the logic.

I think it’s because this would make the imaginary “essence” of a thing not one universal single thing, but a logical connective of multiple nonessential things. That is to say, A and B individually are sufficient but not necessary conditions. The ability to embody at least one of the two is the necessary and sufficient condition.