Key date

A key date is a term used in coin collecting and it refers to a date (or date and mint mark combination) of a given coin series or coin set that is harder to obtain than other dates in the series. A key date coin is usually one with a lower mintage total and it is more valuable than others in the series. Many coin collectors collect coins to fill out a complete set of a series.

There are also semi-key dates in coin collecting. A semi-key date coin is typically one that is not as rare as the key date. The semi-key date often sells for a premium over common date coins, but less than the cost of a key date coin.