Bass note, described in picture, of a C major triadPlay (help·info). The bass note of the triad is also the root of the triad in this scenario; but this is not always the case with triads, such as in the case of inverted triads.
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) While the bass note is often the root or fundamental of the chord, it does not have to be, and sometimes one of the other pitches of the chord will be found in the bass. See: inversion (music).
Examples with bass note in red: C major chord in root position close position (C), open position (C), 1st inversion (E), 2nd inversion (G), and cluster on C (C). Play (help·info)
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.