The term “key date” is a piece of numismatic jargon, used by coin collectors to describe a coin that is “key” to completing the set to which it belongs.
To put this definition in context, here are a few alternative uses of the word “key”:
• a thing that provides a means of gaining access to or understanding something;
• a set of answers to exercises or problems.
• a word or system for solving a cipher or code.
These alternative uses help us appreciate how the term “key date” is used to describe a coin that is fundamental to opening up, or completing a set. If a “key date“ is not included in a set, it will never be considered to be complete.
Key dates are generally coins that have a level of rarity such that collectors always need to compete to get them. They are available so rarely that whenever they do come to market, they are met by a backlog of demand and prices respond accordingly.