Superposition
A quantum system being in a combination of several states at once — not "we don't know which", but genuinely both.
The difference from ignorance is measurable. A coin under your hand is either heads or tails and you happen not to know. A qubit in superposition has amplitudes for both outcomes, and amplitudes can be negative and cancel each other. That cancellation is experimentally demonstrable and has no classical counterpart.
On measurement one outcome remains and the superposition is gone. So it gives no free parallelism: you cannot run every alternative and then read out the best. The craft in a quantum algorithm lies in arranging the interference so that wrong answers have cancelled before you measure.