The dating system used throughout Starfleet and the Federation, a decimal number quoted at the start of log entries in place of a calendar day, month and year.
A stardate is the number by which Starfleet records the date. It is quoted at the head of almost every log entry and is used for everything from duty rosters to service records, in place of the calendars of the Federation's many member worlds.
In the original series stardates are four-digit numbers with a decimal, rising loosely as the series goes on; the second pilot, Where No Man Has Gone Before, is set on stardate 1312.4. The Next Generation introduced five-digit stardates beginning with 41, in which each season of that series, Deep Space Nine and Voyager advances the figure by about a thousand, so that a thousand units roughly match one year.
The films beginning with Star Trek in 2009 use a different form built on the calendar year followed by a decimal, such as 2233.04 for the day the USS Kelvin is lost.