top
In SM, the person who is the one "doing," is the top, as opposed to the bottom, who is being "done to." It's a noun and a verb, too, as in "She was topping me" or "I had a great time bottoming to her."
The terms top and bottom are value-free, unlike the terms dominant and submissive. People often want to distinguish between bottoming and submitting. I'm not so fussy, but there is a subtle difference.
mrnosuch 
nosuch.org