Definition from Usability First
“A prototype that only works by having someone behind-the-scenes who is pulling the levers and flipping the switches. The wizard of oz technique in user testing has a user interacting with an interface without knowing that the responses are being generated by a human, not a computer.”
Going further
I've curated 400+ definitions just like this one in a tiny dictionary. In this glossary you can easily access, search and understand 400+ terms that service designers and other innovators use.