Mockup = low fidelity sketch or design in some graphic design application or on a napkin
Wireframe = layout of the project's structure or storyboard based on the mockups
Prototype = low level functioning project based on the wireframes and mockups for the purpose of testing the project in the environment in which it's meant to exist.
Comment by Homer Gaines, CPACC on "What's the basic difference between Mockup, Prototype and Wireframe?" | Hashnode