A tone cluster is a musical chord comprising at least three adjacent tones in a scale. Prototypical tone clusters are based on the chromatic scale and are separated by semitones. For instance, three adjacent piano keys (such as C, C♯, and D) struck simultaneously produce a tone cluster. Variants of the tone cluster include chords comprising adjacent tones separated diatonically, pentatonically, or microtonally. On the piano, such clusters often involve the simultaneous striking of neighboring white or black keys.

The early years of the twentieth century saw tone clusters elevated to central roles in pioneering works by ragtime artists Jelly Roll Morton and Scott Joplin. In the 1910s, two classical avant-gardists, composer-pianists Leo Ornstein and Henry Cowell, were recognized as making the first extensive explorations of the tone cluster. During the same period, Charles Ives employed them in several compositions that were not publicly performed until the late 1920s or 1930s, as did Béla Bartók in the latter decade. Since the mid-20th century, they have prominently featured in the work of composers such as Lou Harrison, Giacinto Scelsi, Alfred Schnittke and Karlheinz Stockhausen, and later Eric Whitacre. Tone clusters also play a significant role in the work of free jazz musicians such as Cecil Taylor, Matthew Shipp, and Kevin Kastning.

In most Western music, tone clusters tend to be heard as dissonant. Clusters may be performed with almost any individual instrument on which three or more notes can be played simultaneously, as well as by most groups of instruments or voices. Keyboard instruments are particularly suited to the performance of tone clusters because it is relatively easy to play multiple notes in unison on them.

Prototypical tone clusters are chords of three or more adjacent notes on a chromatic scale, that is, three or more adjacent pitches each separated by only a semitone. Three-note stacks based on diatonic and pentatonic scales are also, strictly speaking, tone clusters. However, these stacks involve intervals between notes greater than the half-tone gaps of the chromatic kind. This can readily be seen on a keyboard, where the pitch of each key is separated from the next by one semitone (visualizing the black keys as extending to the edge of the keyboard): Diatonic scales—conventionally played on the white keys—contain only two semitone intervals; the rest are full tones. In Western musical traditions, pentatonic scales—conventionally played on the black keys—are built entirely from intervals larger than a semitone. Commentators thus tend to identify diatonic and pentatonic stacks as "tone clusters" only when they consist of four or more successive notes in the scale. In standard Western classical music practice, all tone clusters are classifiable as secundal chords—that is, they are constructed from minor seconds (intervals of one semitone), major seconds (intervals of two semitones), or, in the case of certain pentatonic clusters, augmented seconds (intervals of three semitones). Stacks of adjacent microtonal pitches also constitute tone clusters.

In tone clusters, the notes are sounded fully and in unison, distinguishing them from ornamented figures involving acciaccaturas and the like. Their effect also tends to be different: where ornamentation is used to draw attention to the harmony or the relationship between harmony and melody, tone clusters are for the most part employed as independent sounds. While, by definition, the notes that form a cluster must sound at the same time, there is no requirement that they must all begin sounding at the same moment. For example, in R. Murray Schafer's choral Epitaph for Moonlight (1968), a tone cluster is constructed by dividing each choir section (soprano/alto/tenor/bass) into four parts. Each of the sixteen parts enters separately, humming a note one semitone lower than the note hummed by the previous part, until all sixteen are contributing to the cluster.

Tone clusters have generally been thought of as dissonant musical textures, and even defined as such. As noted by Alan Belkin, however, instrumental timbre can have a significant impact on their effect: "Clusters are quite aggressive on the organ, but soften enormously when played by strings (possibly because slight, continuous fluctuations of pitch in the latter provide some inner mobility)." In his first published work on the topic, Henry Cowell observed that a tone cluster is "more pleasing" and "acceptable to the ear if its outer limits form a consonant interval." Cowell explains, "the natural spacing of so-called dissonances is as seconds, as in the overtone series, rather than sevenths and ninths....Groups spaced in seconds may be made to sound euphonious, particularly if played in conjunction with fundamental chord notes taken from lower in the same overtone series. Blends them together and explains them to the ear." Tone clusters have also been considered noise. As Mauricio Kagel says, "clusters have generally been used as a kind of anti-harmony, as a transition between sound and noise." Tone clusters thus also lend themselves to use in a percussive manner. Historically, they were sometimes discussed with a hint of disdain. One 1969 textbook defines the tone cluster as "an extra-harmonic clump of notes".

In his 1917 piece The Tides of Manaunaun, Cowell introduced a new notation for tone clusters on the piano and other keyboard instruments. In this notation, only the top and bottom notes of a cluster, connected by a single line or a pair of lines, are represented. This developed into the solid-bar style seen in the image on the right. Here, the first chord—stretching two octaves from D₂ to D₄—is a diatonic (so-called white-note) cluster, indicated by the natural sign below the staff. The second is a pentatonic (so-called black-note) cluster, indicated by the flat sign; a sharp sign would be required if the notes showing the limit of the cluster were spelled as sharps. A chromatic cluster—black and white keys together—is shown in this method by a solid bar with no sign at all. In scoring the large, dense clusters of the solo organ work Volumina in the early 1960s, György Ligeti, using graphical notation, blocked in whole sections of the keyboard.

The performance of keyboard tone clusters is widely considered an "extended technique"—large clusters require unusual playing methods often involving the fist, the flat of the hand, or the forearm. Thelonious Monk and Karlheinz Stockhausen each performed clusters with their elbows; Stockhausen developed a method for playing cluster glissandi with special gloves. Don Pullen would play moving clusters by rolling the backs of his hands over the keyboard. Boards of various dimension are sometimes employed, as in the Concord Sonata (c. 1904–19) of Charles Ives; they can be weighted down to execute clusters of long duration. Several of Lou Harrison's scores call for the use of an "octave bar", crafted to facilitate high-speed keyboard cluster performance. Designed by Harrison with his partner William Colvig, the octave bar is