A tetrad is a group or set of four — the number four treated as a unit, and by extension any four-part arrangement.
The word entered English in the 1650s from Greek tetras (stem tetrad-), "group of four, the number four," which descends from the Proto-Indo-European root **kwetwer-*, "four" — the same source as Latin quattuor and English four. The bare numerical sense survives, but the word is now carried almost entirely by its technical uses, which is why it rarely appears outside a field that has claimed it.
In genetics and cell biology, a tetrad is the four-part structure formed during prophase I of meiosis: two homologous chromosomes, each already duplicated into two sister chromatids, paired along their length. That alignment is what makes crossing over possible, so the tetrad is the physical precondition for genetic recombination rather than merely a stage on the way to it. A second biological sense assigns the word to the four haploid cells a completed meiotic division produces. Chemistry uses it for a tetravalent atom or radical — one with a combining power of four. Music assigns it to a four-note chord, astronomy to four consecutive total lunar eclipses, and general relativity to a frame field of four basis vectors.
Media theory supplies the outlier. The tetrad of media effects is 📝Marshall McLuhan's analytical tool, set out in Laws of Media (1988): four questions asked of any technology — what it enhances, what it obsolesces, what it retrieves that had been obsolesced earlier, and what it reverses into when pushed to its limit. Unlike every other sense of the word, McLuhan's four are not interchangeable members of a set but four distinct pressures acting simultaneously.