something that either partner in a BDSM relationship will not do, or does not like. Basically, a specific preference concerning BDSM play. The submissive's Limits should always take precedence over the Dominant's. Limits should always be discussed and set out before a Scene ever starts. Respecting Limits is not an option, it's a requirement.
In BDSM, this refers to an activity or fetish that a person will refuse to participate in; in D/s, it refers to the same as BDSM, but a much larger scope, extending to everyday life (for instance, refusing to allow a Dominant to choose clothing). Limits are often a topic of discussion between Doms and subs who may potentially engage in a relationship, defining the types and degrees of limits. Hard limits are limits that are firm, and not mutable with circumstance; soft limits are negotiable under certain circumstances.
In SM are the boundaries set by the dominant and the submissive for what to will or to unwill within a scene. Limits must be respected and applied to roles, levels of dominance and submission, time and also activities like whipping and paddling.