I'd call it Drop C# (my band, myself and a few friends of mine just call it that. The 'drop' implies the drop D tuning, but with the bass being whatever the specified note is. Similar to how D Standard is just standard tuning but a whole step down.). That of course could mean different things to different people however, so if you don't know what they call it just call Drop D a half step down.
Thanks I thought you called it that, I was getting Drop C# mixed up with C# standard.
I go with Drop C# my bands play in that tuning. Gives it a little extra mmph on the low end.