1. Historical Marking What does B.C. mean?
The era we now call BC used to be known as "a.C.n.", an abbreviation of "Ante Christum Natum", which is Latin for "before the birth of Christ".
Why the terminology changed from Latin to English is a matter of speculation. In non-English speaking countries, they tended to use the local language: in French, "avant J.C." (before Jesus Christ); in German, "v. Chr. Geb.", an abbreviation of "vor Christi Geburt" (before Christ's birth) Hence, in English B.C. (before Christ)
Historical Marking
What does B.C. mean?

2. AD is an abbreviation of anno Domini Nostri Jesu Christi, Latin for "in the year of our Lord Jesus Christ".
Anno Domini (abbreviated as AD or A.D.) and Before Christ (abbreviated as BC or B.C.) are designations used to label or number years used with the Julian and Gregorian calendars. This calendar era is based on the traditionally reckoned year of the conception or birth of Jesus of Nazareth, with AD counting years after the start of this epoch, and BC denoting years before the start of the epoch.
Era
What does A.D. mean?

3. There is also another version of AD and BC
There is also another version of AD and BC. The years we know of as AD are now to be known as CE, "Common Era", and the years we know of as BC are to be known as BCE, "Before Common Era".
Era
What does B.C.E. mean?