1. noun /ˈdɒmɪnənt,ˈdɑmənənt/
a) The fifth major tone of a musical scale (five major steps above the note in question); thus G is the dominant of C, A of D, and so on.
b) The triad built on the dominant tone.
2. adjective /ˈdɒmɪnənt,ˈdɑmənənt/
Ruling; governing; prevailing; controlling; predominant; as, the dominant party, church, spirit, power.
Wikipedia foundation.