In neurology, dominant refers to the side of the brain where the ability to recognise and create speech resides.

In most people, this is the left side of the brain.

A dominant, in color photography, is a color that came out stronger than it was supposed to, generally to the detriment of its complementary color (not to be confused with color shift).
Usually, when inspecting a color print, you can recognize cyan, magenta, yellow, green, blue and red dominants; recognizing the dominant gets you half way to getting a good color print.
Nearly everybody can tell if "the colors are off", but it is more difficult to guess at the dominant: a useful tool for determining which color is crushing the others is a set of viewing filters, that also helps in correcting the filterpack.

Though the exact terminology can be mixed and muddied somewhat, in sex, a Dominant (often capitalized) is the one who is in control in a Dominance & submission relationship, which can include elements of sadomasochism and/or bondage (but doesn't have to).

For Dom/mes, having someone surrender themselves to you in an act of submission is an incredibly powerful experience... to hold their well-being in your hands, to know that the submissive trusts you to honorably wield the power they have given you, and to not abuse it... I've not the proper words for it.

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.

Dom"i*nant (?), a. [L. dominans, -antis, p. pr. of dominari: cf. F. dominant. See Dominate.]

Ruling; governing; prevailing; controlling; predominant; as, the dominant party, church, spirit, power.

The member of a dominant race is, in his dealings with the subject race, seldom indeed fraudulent, . . . but imperious, insolent, and cruel. Macaulay.

Dominant estate ∨ tenement Law, the estate to which a servitude or easement is due from another estate, the estate over which the servitude extends being called the servient estate or tenement. Bouvier. Wharton's Law Dict. -- Dominant owner Law, one who owns lands on which there is an easement owned by another.

Syn. -- Governing; ruling; controlling; prevailing; predominant; ascendant.

© Webster 1913.

Dom"i*nant, n. Mus.

The fifth tone of the scale; thus G is the dominant of C, A of D, and so on.

Dominant chord Mus., the chord based upon the dominant.

© Webster 1913.