a scarce date required to complete a collection, usually more difficult to find and afford.

Among the scarcest (and therefore most expensive) members of a coin series, e.g. the 1909-S VDB Lincoln cent or 1916-D Mercury dime

a scarce date usually required to complete a coin collection

Key Dates refer to a coin that is the very rarest coins in a series and therefore the most expensive coins. Key Dates even in low grades are very valuable and highly sought after.

a scarce date that is often hard to find to complete a collection.

Slang usually indicating the rarest (and therefore most expensive) date-and-mint of a particular coin series.

An extremely low mintage and /or certified population, which results in higher collector interest. Nearly every coins series has one or two issues with such dates. For example, the 1916-D in the Mercury dime series or the 1911-D in the $2-1/2 Indian series.

Used in coin collecting; a key date is 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.