A ‘top’ gives the pain or sensation which a ‘bottom’ wishes to receive, so for example in a spanking the ‘top’ will administer the spanking and the ‘bottom’ will be spanked. The word can also be used as a verb (as in ‘topping’ someone, meaning to give them pain or sensation).The words ‘top’ and ‘bottom’ are often used interchangeably with ‘dominant’ and ’submissive’ but they are not synonymous. In relations between a dominant and a submissive there is always an exchange of power, where the submissive agrees to submit to the will of the dominant. That same surrender isn’t required or implied in a session between a ‘top’ and a ‘bottom’. Others sometimes equate the words to sadist (top) and masochist (bottom). Definition from Sasha White.