Drop C is a tuning for guitar and bass where the lowest string is tuned down by two full steps and all other strings are tuned down by one step, so the outcome is CGCFAD.
It is not to be confused with C Sharp Standard or "Black Sabbath" tuning which means that all strings are tuned down by one and a half steps.
The biggest advantage of Drop C is that you can use a single finger (instead of two or three) to play powerchords on the lowest three strings. This technique is known as "chocolate finger" among some guitarists.
Children of Bodom used Drop C on some songs and albums.
System of a Down used Drop C tuning on their first three albums.
Black Sabbath never used Drop C, they tuned their instruments to C#.
An alternate tuning for the guitar. The strings are tuned CADGBE as opposed to EADGBE. Drop C is more commonly (and mistakenly) used to refer to "Dropped D Tuned Down One Whole Step"; CGCFAD.
Bill: Dude, do you know any songs in Drop C?
Bob: No, but I know some in Dropped D Tuned Down One Whole Step.
Bill: Oh, bummer.