What are clusters in musical terms?

Clusters refer to playing several adjacent keys on a keyboard instrument simultaneously to produce a full, rich, and dissonant sound. Clusters can consist of two or more notes and can be played on any keyboard instrument, such as piano, organ, or synthesizer. These clusters can be used for various musical purposes, including creating atmospheric effects, adding tension or release, and evoking specific emotions. Cluster playing techniques were popularized in the 20th century by composers like Béla Bartók and György Ligeti.