12. What is "Proof Coin"?

"Proof Coin" originally means a coin which is especially produced for comparison with circulating coins (as it were "Standard Coin"). Generally speaking, it is a brilliantly polished numismatic coin. The proof coin is carefully produced with special techniques.
For example, the surfaces of coining dies are polished with much care and coins are stamped twice in order to make the relief-work deeper and clearer. Proof coins produced in our mint and sold in Japan, have a mirror like luster in the hollow area of the surface but have no luster in the highly raised area of the surface. The upper part is distinguishable from the lower part. Some foreign countries produce the proof coin with its surface all mirror-like brilliance.

*Informal translation
For accuracy and nuances, see
The law concerning the unit of currency, issuance of coinage and so forth. (1982, law, No.42)
Article 10 The Japan Mint sells coins authorized by the finance minister stipulated below.
2 Coins which are produced with special techniques and have luster on the surfaces.