Bionics means the replacement or enhancement of organs or other body parts by mechanical versions. Bionic implants differ from mere prostheses by mimicing the original function very closely, or even surpassing it.
Bionics' german equivalent "Bionik" takes a somewhat broader scope in that it tries to develop engineering solutions from biological models. This approach is motivated by the fact that biological solutions will always be optimized by evolutionary forces. A classical example is the development of dirt- and water-repellent paint (coating) from the observeration that the surface of the lotus flower plant is practically unsticky for anything.
While the technologies that make bionic implants possible are still in a very early stage, a few bionic items already exist, the best known being the cochlear implant, a device for deaf people. Some versions come quite close to "normal" hearing; they can even work better than natural ears at background noise filtering.