In music, the dominant is the fifth scale degree of the diatonic scale, called “dominant” because it is next in importance to the tonic, and a dominant chord is any chord built upon that pitch, using the notes of the same diatonic scale. The dominant is sung as so in solfege. The dominant function (diatonic function) has the role of creating instability that requires the tonic for resolution.

https://en.wikipedia.org/wiki/Dominant_(music)