In BDSM, the term power exchange refers to a relationship or activity in which the submissive gives to the dominant partner power and authority over the submissive's body in exchange for the submissive's happiness and health. The details of the arrangement would be negotiated and set out in a formal contract and can be for any duration, according to the agreement of the participants, ranging from a single scene, to a proscribed period of hours, days or weeks, to a 24 hour a day open-ended agreement with no termination date.
On a psychological level, most BDSM roleplay involves power and dominance, whereby one person willingly hands over personal autonomy to another. This can range from addressing another person as "master" or "mistress" for a ten-minute scene, to a witnessed, formal collaring with a lifelong agreement which micro-manages the submissive's life.
The latter is often referred to as total power exchange or TPE or 24/7 or 24/7/365.
In "safe, sane and consensual" BDSM, power exchange is always negotiated. Before play, the participants would discuss their physical and psychological limitations, establish safewords and work out what will happen.
A submissive is a person who submits or potentially submits to another. Within a BDSM-only context, submissive is sometimes synonymous with bottom. Submissives can vary in how seriously they take their position, training, and situation. Reasons for this include relief from responsibility, being the object of attention and affection, gaining a sense of security, showing off endurance or working through issues of shame.
A dominant is a person who exercises the power to take control of a person or situation through usage of some means (such as physical, mental, financial, etc.) on a regular basis; the gender specific titles being dom for a man, domme or dominatrix for a woman. Reasons for this include demonstrating skill and power, having ownership of another person, being the object of affection and devotion.
In most power exchange as referred to in a BDSM scene, there are limitations on the power the dominant has over the submissive, include things such as safewords, time limits, or explicitly negotiated understandings of what is allowed.
In some cases, "topping from the bottom", or the attempt by a submissive to covertly control the top, is considered poor practice within lifestyle BDSM and power exchange. However, in the instance of interrogation scene play, such cues from the submissive are necessary in order to have a successful experience.
The term power exchange translated into Latin is "visvicis", "vis" meaning power and "vicis" meaning exchange.
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