[mrxd]

That's not how its used in UX design. A mockup is essentially a screenshot: non-interactive, visually accurate. A prototype is an interactive simulation of the system. The term encompasses simulations which are very far from the final product, like creating interfaces using hand-drawn pieces of paper and sticky notes which are operated by a human acting as the system.

I find that it's helpful to evaluate ideas on their merits instead of quibbling over terminology.

[thatthatis]

Chicken chicken chicken chicken chicken[chicken], chicken chicken chicken (chicken chicken chicken chicken).

Chicken: http://isotropic.org/papers/chicken.pdf

[seanmcdirmid]

If we go by intent: a mockup is used to get project buy-in and early feedback from stakeholders; design is still at the planning stage. A prototype is an abstraction of the system used to validate interaction features during design.

Getting terminology right is important from "does X solve problem Z, and are X and Y the same thing when it comes to solving problem Z."