A circuit which limits the rate at which a signal can increase or decrease in value. This circuit, or a software simulation of it, is most often used to produce portamento by processing a control voltage which is subsequently used to control the frequency of a VCO. In the analog synthesizer world, two types of lag processor exist: the more common type is basically a low pass filter with a below-audio cutoff frequency. It exhibits an exponential transfer curve; that is, if the gap between the input and output voltage is large, the ouptput will rise/fall more rapidly, and if the gap is small, the output rises or falls slowly. A second, less common type is known as "slew rate limiting"; it simply imposes a hard upper limit on the slope of the output.
Digital simulations of lag processing usually fall into two categories: "constant rate", in which the slew from one frequency to another proceeds at a constant number of half-steps in an interval of time, and "constant time", in which the slew increases or decreases depending on the interval to be covered, so that the amount of time it takes to transition from one note to the next is always the same.