B.C. = Before Christ, suffixed to years counted backwards from 1 B.C. In modern academic style the form B.C.E. is preferred, standing for Before Common (or Civil) Era. The corresponding positive forms are A.D. for Anno Domini and C.E. for Common (or Civil) Era. In all cases it is very common to omit the full stops: BC, BCE, AD, CE.
The idea of marking dates as B.C. was not introduced until 1627, over a thousand years after A.D. was invented. The French Jesuit astronomer Denis Petau (1583-1652), also known Latinized as Petavius and nicknamed Prince of Chronologers, marked dates this way in lectures at the Collège de Clermont in Paris.
He is also therefore responsible for our peculiar modern device of counting backwards. Before him pre-Christian dates were named either from the creation of the world (according to various people's computations from the Bible) or by using the classical starting-years, the first Greek Olympiad in 776 BCE and the foundation of Rome (the AUC date) in 753 BCE.
Dionysius Exiguus set the year 1 AD as the year of the incarnation (25 March) and birth (25 December) of Jesus. Dionysius had no concept of the number zero; but Petavius did, and also negative numbers, but chose not to use them. He named the preceding year not 0 but 1 BC, which is logically correct in the sense of its being the first year before Christ, but makes calculations harder.
He would have been writing in Latin, and using Ante Christum and A.C., though it would come out the same in French.
Modern astronomers do use zero and negative numbers to simplify their calculations. The years 1 BC and 2 BC are designated the years 0 and -1.