A person who prefers to be the one penetrating their partner, rather than being penetrated.
This is where one person is the dominant partner, in that they are the one in control, who actually penetrates the other. However, it doesn't mean they are more 'active' than the other, but for political correctness, the term is preferred over the more common 'top' term.
In BDSM culture, the 'top' is the person who controls the session, who administers and/or initiates the sexual activity.
Should be noted, that a Top is not always the one who does the penetrating, but is the one in control, in charge.
Like other terms, there is a wide scope of actions, that are associated with the meaning of the word. A 'top' is generally used to describe the person who enters or penetrates another. However, in some situations, this is simply referencing a person who does so at the wishes of another, in the manner prescribed.
Mostly in BDSM, the position of being on top (in a literal sense) does not conform to the strict interpretation of the word.
Service Top for example, refers to one who does the actions associated with being a top, but at the direction of the dominant person, who can be the one receiving the penetration.
Confusing, but in most vanilla type sex the top is the one who does the penetrating. They are also referred to as being the 'active' party, which has no bearing on the meaning of the word.
Noteworthy: In BDSM, a 'Top' can simply be the person who supplies the stimulation for the other person, but not actually be the one in control of the session.
In addition, the phrase 'topping from the bottom' refers to the one in control, but who also is the one receiving the anal penetration.