before vowels tel-, word-forming element meaning "far, far off, operating over distance" (also, since c. 1940, "television"), from Greek tele "far off, afar, at or to a distance," related to teleos (genitive telos) "end, goal, completion, result," from PIE root *kwel- (2) "far" in space or time.

before vowels thalass-, word-forming element meaning "sea, the sea," from Greek thalassa "the sea" (in Homer, when used of a particular sea, "the Mediterranean," as opposed to ōkeanos), a word from the Pre-Greek substrate language. In Attic Greek thalatta, hence sometimes thalatto-.

before vowels thec-, word-forming element used in botany and zoology with the sense "case, capsule," from Latinized combining form of Greek thēkē "case, receptacle," from PIE *dhek-, suffixed form of root *dhe- "to set, put."

often thero-, word-forming element meaning "beast," from Greek thēr "wild beast, beast of prey," from PIE root *ghwer- "wild beast." Also therio-, from Greek thērion "wild animal, hunted animal."