The original meaning of a proof was a coin struck to 'prove' the dies. More recently it has come to mean a coin struck two or more times with polished dies on a planchet which also has been polished. Thus a coin struck only once does not qualify as a proof. The US Mint sells proof coins in sets that come in hard plastic cases. You may still find a proof coin in circulation, discarded by a collector or dealer because of some defect. US proof coins are legal tender.