Experience prototpying involves not just artifacts in a vacuum, but the interaction over time between those artificts, the user, and the context in which they will be used. This may include "social circumstances, time pressures, environmental conditions, etc." (Buchenau et al. 2000)
Buchenau and Suri define experience prototype as "any kind of representation, in any medium, that is designed to understand, explore or communicate what it might be like to engage with the product, space or system we are designing". But say they'll "focus on the methods and techniques which support active participation to provide a relevant subjective experience".
I would say this latter part seems to be a better definition to me. A prototype that provides a "relevant subjective experience". That's somewhat tautological, but not really, because we can use a dumber definition of experience than before. Basically, we just want people to live through something that bears a relevant subjective similarity to living through the real thing.
That still leaves open the question, what is "relevant"?