Any bird that has been taken from its parents any time before fledging and fed by a human surrogate.
People selling baby birds often define the term more loosely than people looking to buy baby birds.
It does not recognize a difference between humans and birds. Imprinted birds often prefer the company of a human over the company of another of its own species. Not all hand-fed birds are imprinted. Just feeding them is usually not enough. There are different levels of bonding.