In music theory

Music theory

Music theory is the study of how music works. It examines the language and notation of music. It seeks to identify patterns and structures in composers' techniques across or within genres, styles, or historical periods...

, the bass note of a chord

Chord (music)

A chord in music is any harmonic set of two–three or more notes that is heard as if sounding simultaneously. These need not actually be played together: arpeggios and broken chords may for many practical and theoretical purposes be understood as chords...

 or sonority

Simultaneity (music)

In music, a simultaneity is more than one complete musical texture occurring at the same time, rather than in succession. This first appeared in the music of Charles Ives, and is common in the music of Conlon Nancarrow and others....

 is the lowest note

Note

In music, the term note has two primary meanings:#A sign used in musical notation to represent the relative duration and pitch of a sound;#A pitched sound itself....

 played or notated. If there are multiple voices it is the note played or notated in the lowest voice. While the bass note is often the root

Root (chord)

In music theory, the root of a chord is the note or pitch upon which a triadic chord is built. For example, the root of the major triad C-E-G is C....

 or fundamental of the chord, it does not have to be, and sometimes one of the other pitches of the chord will be the root. See: inversion (music)

Inversion (music)

In music theory, the word inversion has several meanings. There are inverted chords, inverted melodies, inverted intervals, and inverted voices...

.
In pre-tonal

Tonality

Tonality is a system of music in which specific hierarchical pitch relationships are based on a key "center", or tonic. The term tonalité originated with Alexandre-Étienne Choron and was borrowed by François-Joseph Fétis in 1840...

 theory (Early music

Early music

Early music is generally understood as comprising all music from the earliest times up to the Renaissance. However, today this term has come to include "any music for which a historically appropriate style of performance must be reconstructed on the basis of surviving scores, treatises,...

), root notes were not considered and thus the bass was the most defining note of a sonority.

So in the chord "C", (C,E,G), the bass note is the note "C".