In music theory, the bass note of a chord or sonority is the lowest note played or notated. If there are multiple voices it is the note played or notated in the lowest voice (the note furthest in the bass.)
Three situations are possible:
In pre-tonal theory (Early music), root notes were not considered and thus the bass was the most defining note of a sonority. See: thoroughbass. In pandiatonic chords the bass often does not determine the chord, as is always the case with a nonharmonic bass.