A coin made with a polished die or using some other special method. It may be struck twice to make the images clearer. Proofs generally do not circulate, but are made for collectors or for sale as illiquidassets.
For example, if a job requires eight proofs, and it wasn't quoted on as "proofed to satisfaction," those five additional proofs and progs (progressives) could cost two to three times more than the whole job.