plural tesserae, "small, square piece of stone," 1650s, from Latin tessera "a die, cube, square tablet with writing on it" used as a token or ticket, from Ionic Greek tessera, neuter of tesseres (Attic tessares) "the numeral four" (from PIE root *kwetwer- "four").
before vowels actin-, word-forming element meaning "pertaining to rays," from Latinized form of Greek aktis (genitive aktinos) "ray of light, beam of light; spoke of a wheel;" a word of unknown etymology. It is perhaps cognate with Sanskrit aktuh "light, ray," Gothic uhtwo "dawn, daybreak," Lithuanian anksti "early" [Beekes].
<a href="https://www.etymonline.com/word/tesseract">Etymology of tesseract by etymonline</a>
Harper, D. (n.d.). Etymology of tesseract. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/tesseract