Top or dominant is the label used to describe a partner who takes the active or controlling role within a BDSM scene, or within a BDSM relationship context. The function of tops and dominants is similar, and in many cases overlaps. For this reason, the terms are used interchangeably in some discussions, although there are differences between the two.
Top typically refers to the individual who is the physical actor within the BDSM context: i.e. they are the one applying the techniques of a sexual/sensual/psychological activity – such as in flogging, bondage, servitude, or humiliation – upon the bottom partner(s).
A dominant typically refers to the individual who has actual control of the BDSM context/relationship, exercising authority or power over (a) submissive partner(s). A dominant which exercises control over a large percentage of a submissive's day-to-day life, or within a formal framework of rules and rituals, is sometimes referred to as a master or mistress, although the use of these terms and consensus as to their meaning, is far from universal.
Positions often overlap, with the top also being the dominant, but this is not always the case. Someone who is "topping" may be doing so at the request, or even the direction, of the bottom partner(s). In such a case, the dominant's function would reside with the bottom(s). Tops who act within this kind of relationship dynamic are sometimes called a service top. A bottom who has dominance over the activities or the relationship is said to be topping from the bottom, even though they are really expressing dominance from the bottom.
Within communities of lifestyle BDSM devotees, there exists a widespread prejudice against both service tops and bottoms that top from the bottom. Both are considered by many to be failing to achieve a "proper" BDSM relationship dynamic – especially if the partners are purported to be trying to achieve dominant-top/submissive-bottom relationship. While it is also theoretically possible that a dominant would not act as a top, and thus have no expression of their control through kink or fetish based activities, many would argue that such relationships would fall outside of a BDSM context, as it lacks any eroticization of the exercise of control.
The range of allowed activities of the top, or the control of the dominant over a partner, is seldom absolute, often operating within a set of defined limits. A common means that bottoms or submissives use to signal a top or dominant partner that their limits are being approached, pushed, or even crossed is the use of safewords. Extreme forms of submission or the practice of edgeplay can remove the safeword option from the bottom or submissive, although this somewhat risky situation is entered into with the consent of the bottom or submissive.
Top or dominants who are also comfortable assuming a bottom or submissive role are referred to as switches.