Drop C tuning can be used to refer to any of many different alternative guitar tunings:
CGCFAD – This can be described as "standard D tuning with a dropped C", or "drop D tuning transposed down a whole step", where the entire guitar is transposed to the key of D, then the low D is moved down an additional whole step to C. This variation is a common tuning for many different kinds of rock and metal bands.
CGDGBE – Dropping the low E string to a C, and the A string to a G, to make a fifth chord. The rest of the strings remain the same
CADGBE – This is achieved by taking standard tuning (EADGBE) and dropping the low E to a C. All other strings remain the same.
^Aledort, Andy (7 April 2010). "Atreyu: Prior Convictions". Guitar World. Retrieved 23 November 2011. "...drop C tuning, which is like drop D but with all of the strings down a whole step [low to high: C G C F A D]. Drop C sounds a little more brutal."