[Angrist]

The original term deus ex machina comes from Greek theatre. Whenever the plot was at a dead point or the writers couldn't come up with anything interesting, they'd introduce a God (deus) that suddenly changed something. They lowered him onto the stage with a crane (machina).

So the term deus ex machina can apply to persons, story elements, plot twists, etc.
Think of somebody returning to the screen, even though you watched him die in one episode. Or a bit of extra info that changes the whole story. Random stuff like that.
Or a love interest suddenly turning out to be the daughter of your arch enemy.

The auto pilot of the Bat (hey, I remembered the name!) turning out to have worked all along, even though you saw Batman sitting in the Bat when he flew out to sea is another good example.

Anyway, that's what I was taught what deus ex machina means.