Pavane is quite right, "dominant" means "5th note of the scale" (like "tonic" means 1st and "subdominant" means 4th).
The well-known "dominant 7th" chord type is simply the type of chord you get by adding a 7th to the dominant (V) chord of a MAJOR scale (or V of harmonic minor).
In G mixolydian mode, however, the true "dominant 7th" chord is Dm7 - because Dm is the dominant chord. But that's gonna confuse a whole lot of people! Best just call it the "v" ("five") chord, or even "minor five" to be clear.
IOW, the phrase "dominant 7th" is ambiguous, and depends on context.