From the DC8 AMM (the only one we have on computer that has a CSD installed):
A constant speed drive (CSD) transmission is installed on each engine to provide a means of driving an AC
generator at a constant speed regardless of variations in engine operating speeds and applied electrical loads. The transmission is located on the forward section of the engine accessory drive case, at the lower centerline of the engine. A quick attach-detach (QAD) kit is used to secure the transmission to the engine accessory drive case.
The CSD and IDG are transmissions. They take an input and modify it into a usable output (in this case a constant RPM, regardless of input speed).