MacGuffin
A quest object whose exact nature barely matters: it exists to motivate characters and launch the plot.
The MacGuffin — a term popularized by Alfred Hitchcock — is the object everyone pursues whose precise content is indifferent: microfilm, briefcase, secret formula. Its function is purely dynamic: giving characters a reason to run, betray, clash. Hitchcock liked pointing out that the audience never needs to know what's inside.
The pure MacGuffin (the briefcase whose contents we never see) owns its emptiness with elegance. But the boundary is instructive: as soon as the object gains intrinsic meaning that bears on the theme — Tolkien's Ring, corrupting its bearer — it's no longer a MacGuffin but a symbol. Knowing which one you're writing spares many useless explanation scenes.
Example
The briefcase in Pulp Fiction: we never learn what's inside, and it changes nothing.