The term Proof (always capitalized in numismatic usage) refers to a manufacturing process which results in a special surface or finish on coins made for collectors. Most familiar are modern brilliant Proofs. These coins are struck at the Mint by a special process. Carefully prepared dies, sharp in all features, are made. Then the flat surfaces of the dies are given a high mirror-like polish.
Specially prepared planchets are fed into low-speed coining presses. Each Proof coin is slowly and carefully struck more than once to accentuate details. When striking is completed the coin is taken from the dies with care and not allowed to come into contact with other pieces. The result is a coin with mirror-like surface. The piece is then grouped together with other denominations in a set and offered for sale to collectors.
Reproduced with permission from The Official American Numismatic Association Grading Standards for United States Coins, 6th edition, © 2005 Whitman Publishing, LLC. All Rights Reserved.
Examples: This proof coin has extremely sharp details, cameo contrast and brilliant fields.