"Before Christ"
All that happened more that
2004 years ago, at time of writing, is considered BC. The date
system comes from the spread of
christianity and the desire to use the birth of Christ as the standard starting point for years. Anything happening after 1BC is refered to as AD (Anno Domini).
However, the monks that tried to work when the birth of christ was did so over 1000 years ago, and were missing some documents. These documents were discovered in the 20th century, and 'twas found the birth of christ to be four years before the dating system.