A dominant is a very specific idea in functional harmony. The dominant's job is to take you back to the tonic. Why a dominant has so much pull in functional harmony is that a dominant chord utilizes the leading tone along with having the common tone with the tonic chord which is the dominant itself.
In terms of scale degrees the dominant is the scale degree where you build chords that tonicizes the tonic of that scale. Naturally, when you build a 7th chord off of G in C Major you get a G7 which by no accident is known as a dominant 7th. A C7 is a dominant chord and its job is to take you to the tonic of F or tonicize F.
