For the 21st century resident, please see B.C..
BC was an era of time on Earth.
Notable years in that time period included 3834 BC, the year in which Akharin survived what ought to have been a fatal wound, and 280 BC, the time of the building of the Lighthouse of Alexandria by Ptolemy II. (TOS: "Requiem for Methuselah"; VOY: "11:59")
BC stands for "before Christ".
In secular publications, like the Star Trek Encyclopedia (4th ed., vol. 1, p. 74), BC has been replaced by the dating reference system BCE (Before Common Era).