1. Dark current is the relatively small electric current that flows through photosensitive devices such as a photomultiplier tube, photodiode, or charge-coupled device even when no photons are entering the device; it consists of the charges generated in the detector through heat,when no outside radiation is entering the detector .Dark current is due to the random generation of electrons and holes within the depletion region of the device.
2. Dark current is unwanted current or signal in a photodetector in the absence of incident light, resulting from thermally excited electrons or leakage of current along the current path
https://en.wikipedia.org/wiki/Dark_current_ (1.); http://www.thefreedictionary.com/dark+current (2.)