Discipline is a BDSM practice in which dominant(s) have submissive(s) obey rules; sometimes the dominant punishes the submissive for failure to follow the rules. All this must be done consensually.
Punishments in Discipline
Punishment can be physical (such as whipping or piercing) or psychological (such as public nudity where legal) or a combination of both (such as torture bondage).
The goal of discipline is to teach the submissive how they should behave as well as the consequences that may arise as a result of breaking the rules of behavior; by undergoing punishment if they break the rules, they learn self-restraint and become better subs. The punishment is generally related to the mistake, and is generally proportionate to the severity and frequency of the mistake. For example, a punishment for speaking out of turn for the first time may be a simple restraint (such as being silenced using a gag). Similarly, the punishment for a submissive getting an erection when that is against the rules can be a chastity cage.
The submissive may also be given the option of choosing his/her own punishment from among options. For example, for a minor mistake done repeatedly, the punishment could be an option to be either caned a few times (physical punishment) or be paraded nude in public with a pet animal leash where such is legal (mental punishment). Or a punishment for a severe rule breaking could involve a choice of whip or cane as well as for body part (such as back or buttocks), on which to receive 50 hard strokes.