From AMS Glossary
That type of gaseous electrical discharge in which the charge transfer occurs transiently along a relatively constricted path of high ion density, resulting in high luminosity.
It is of short duration and to be contrasted with the nonluminous point discharge, the corona discharge, and the continuous arc discharge. The exact meaning to be attached to the term "spark discharge" varies somewhat in the literature. It is frequently applied to just the transient phase of the establishment of any arc discharge. A lightning discharge can be considered a large- scale spark discharge.