Date Certain
A term identifying the date on/by which the specified actions of a contract can be reasonably completed. This date is important, as it is generally considered legally binding.
Investopedia Commentary
The third Friday of each month is an example of a date certain for option expiry. Remember, options (and in fact all derivatives) are simply contracts between two parties.
Related Links
Options Basics Tutorial
See also: Derivatives, Expiration Date, Option, Settlement Date, Sum Certain, Triple Witching Hour