In science, "bc" and "ac" usually refer to "before Christ" and "anno Domini" (Latin for "in the year of the Lord"). These are used in the BC/AD dating system, a method of chronological dating that places events in relation to the traditional year of the birth of Jesus Christ.

* BC: Used for dates before the traditional year of Jesus Christ's birth. For example, 3000 BC is 3000 years before the year 1 AD.

* AD: Used for dates after the traditional year of Jesus Christ's birth. For example, 1066 AD is 1066 years after the year 1 AD.

However, it's important to note that "bc" and "ac" can sometimes have different meanings depending on the specific context.

Here are some other possible meanings:

* BC: Could also stand for "British Columbia", a Canadian province.

* AC: Could stand for "Alternating Current", a type of electrical current.

To understand the meaning of "bc" and "ac" in a specific situation, pay close attention to the context and the surrounding information.