A maintenance spanking is an undeserved spanking given as part of a consensual agreement by an authority figure as a way for him/her to maintain dominance over the spankee. It may also be given to remind the spankee of what the result of bad behavior feels like, or because the authority figure believes that the spankee has not been spanked for recent misbehavior only because he/she hasn't been caught.
Maintenance spankings are typically given in an environment where spanking is the most common form of discipline, most frequently a domestic setting. The spanking may be part of a ritual, or set at a regular time (say, once a week). They are usually not given with the severity of a punishment spanking, although they may evolve into one if the spankee displays too much resistance at the idea of receiving a spanking without committing any misbehavior to earn it. While a spanker may commence the ritual, it's often the spankee who is needing the maintenance spanking.