BC - Before Christ
Before Christ is used to date events before the birth of Jesus. AD and BC are designations used to label or number years used with the Julian and Gregorian calendars. There is no zero year in this scheme, so the year 1 BC is immediately followed by the year AD 1.
This system was created to make the birth of Jesus Christ the dividing point in time. However, when A.D/B.C system was actually calculated, they made a mistake in counting the number of years by a pinpoint. It was later discovered that Jesus was actually born around 4-6 BC, not AD 1.