BC

BC or B.C. was an initialism used to designate an Earth year's position relative to year 1. Hence, 1 BC was 1 year before 1 AD, there being no year Zero. The term "BC" thus designated a negative number; 100 BC was 99 years before 1 BC. Its positive counterpart was AD.

Behind the scenes
An inherently Christian designation, BC is not generally used in calendars of the traditionally non-Christian world. Consequently, some people use BCE (meaning Before Common Era) for BC. CE (meaning Common Era) as a substitute for AD. These largely cosmetic replacements are thought to avoid religious offence. However, Doctor Who fiction has typically remained loyal to the BC/AD convention. Even as late as the RTD and Moffat eras, dialogue and on-screen graphics prefer the Christian convention. (DW: The Fires of Pompeii, The Pandorica Opens, A Good Man Goes to War)