The inability of a voltage controlled oscillator (or voltage controlled filter) to remain in tune over time. The most common cause of this is the exponential converter circuit that most VCO and VCA circuits use to convert a volts/octave control voltage input to the volts/Hz signal that the core circuit requires. The conventional exponential converter circuit uses a transistor in a way that is inherently sensitive to temperature, due to the semiconductor physics involved. In order to be stable, the circuit requires temperature compensation to accommodate variations in temperature due to ambient conditions and heat buildup inside the synth's case. (Most such circuits employ a tempco that is in physical contact with the case of the expo conversion transistor.) Proper design of the compensation circuit is essential to minimizing drift.
One of the aims of a DCO circuit is to minimize the dependency between the expo-converted control voltage and the output frequency. Several approaches to this have been used, but the most common involves some variation of capturing the control voltage in a digital form and then using this to set a timer, which takes the place of the comparator circuit in the VCO core.