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A Dyson sphere is a hypothetical megastructure first described in 1960 by the physicist Freeman Dyson in a short paper published in the journal Science entitled "Search for Artificial Stellar Sources of Infra-Red Radiation". This is often interpreted as an artificial hollow sphere of matter around a star (as the diagram depicts). This perception, which is often used in science fiction, television shows such as Star Trek, etc., is a misinterpretation of Dyson's original concept. Dyson used the term "shell", not "sphere" in his original paper. Dyson also clearly states in his comments to the letters resulting from his original article, "A solid shell or ring surrounding a star is mechanically impossible. The form of 'biosphere' which I envisaged consists of a loose collection or swarm of objects traveling on independent orbits around the star." The term "Dyson swarm" has recently come into use as a way to make this distinction clearer. The primary motivation Dyson proposed for constructing a Dyson swarm is to allow a civilization to capture nearly all of a star's energy - by contrast, less than one part per billion of the Sun's energy even hits Earth.

Although Dyson is credited with being the first to formalize and popularize the concept of the Dyson sphere, Dyson himself got the idea in 1945 from a science fiction novel titled Star Maker by Olaf Stapledon. The original proposal by Dyson did not go into much detail about how a Dyson sphere would be constructed, focusing instead on the more fundamental issue of how an advanced civilization could expand its energy production to the maximum possible for a given solar system. Such a civilization would be classified as a Type II civilization under the Kardashev classification scheme developed by the astronomer Nikolai Kardashev. It is useful to understand that all KT-II civilizations (i.e. all civilizations which may have constructed a Dyson swarm) are unlikely to emit light in the visibile spectrum (because they are harvesting it for energy production). So attempts at SETI involving pointing receivers at visibile stars are inherently targeting civilizations which have not reached the KT-II level.

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