In music, the dominant is the (usually, unless you are *very* strange indeed) fifth chord in the key. In other words, in the key of C Major, the chord would be G Major.
The dominant is almost always Major. This is because Major chords are typically considered more resolute and have "less entropy" (to introduce some pseudoscience to the topic) than Minor or other chords.
Some scales, like the diminished scale, do not even have a proper dominant. This is because the fifth chord is diminished and is on a diminished degree of the scale. Dominants can only be Major, Minor, or a variation thereof.
Here is a list of strengths of various dominants:
Weak | Minor
- | Major M7
- | Minor m7
- | Major
Strong | Major m7
To a lesser extent, any chord can accomplish the role of a dominant. Typically this is the leading tone, which is the chord one half-step below the tonic, or the first chord. The subdominant, or the fourth chord can also somewhat become a dominant.
But this is just child's play. There are also altered dominants, secondary dominants, and that whole modulation deal.