Ian Goodfellow in a Quora answer defines zero-shot learning as the following:

Zero-shot learning is being able to solve a task despite not having received any training examples of that task. For a concrete example, imagine recognizing a category of object in photos without ever having seen a photo of that kind of object before. If you’ve read a very detailed description of a cat, you might be able to tell what a cat is in a photograph the first time you see it.