As an example: Phocion, Athenian statesman and general (d. c. 318 BC) Have seen this in some textbooks and I wasn't able to google the answer.
It means "death circa." Phocion "the Good" lived c. 402 BC – c. 318 BC.
That's a short form of expressing the years a person lived. In this case, they give no birth date, but say that he died (d.) around (c.=circa) the year 318 BC.