In coin collecting, a key date refers to a date (or date and mint mark combination) of a given coin series or set that is harder to obtain than other dates in the series. The next level of difficult to obtain coins in series are often referred to as semi-key dates or simply semi-keys.
For example, the 1969 Australian 2 Cent Coin at only 12.9 million minted.
What makes a coin a "key" is somewhat more complicated because there are many issues that are easy to obtain in lower grades and very hard to find in higher ones. Collectors who want a series in near uncirculated condition may find some date/mint combinations extremely rare. This is because low mintage coins were frequently kept by collectors and saw little or no circulation, while common dates were rarely saved and typically became heavily worn.
Please note, I have not included Low Mintage Key date or Semi-Key date circulating commemorative coins in this section.
Though many circulating commemorative coins fit into this section (and many don't), I have kept the two separate.
Circulating Commemorative Coins can be found here: