Reading the answers, it seemed to me that everyone was getting bogged down in the technical aspects, and was missing the essential point of the term.
So, I added my own answer, writing:
Polymorphism is a $10 word for a $1 idea - that when I ask for something to be done, I don’t care how it is achieved as long as the end result is appropriate. As long as the service is provided correctly, I don’t care about the implementation.
What do you think of my definition?