A prototype is a model or early version of a device or product. Different kinds of prototype may be used at dfferent points during the UX design process A low-fidelity prototype may use blue foam or glued cardboard for physical devices or paper sketches for screens, and is partcularly useful early in the design process to get rapid feedback about ideas from users or clients. High-fidelity prototypes may use pixel-perfect screens, sometimes with some underlying functionality working for specific interaction paths; these may be used for user evaluation later in design.
Used in glossary entries: blue foam, high-fidelity prototypes, low-fidelity prototype, sketch
Links:
alandix.com: 
Low-fidelity (left) and high-fidelity (right) prototypes of a mobile phone; real phone in centre. From TouchIT – Chapter 20: Prototyping and Tool Support
Sketches of screens for use in low-fidelity prototype