Ancient Athenian legal practice gave theta an unusually grim second job, distinct from its ordinary sound value covered elsewhere in this atlas: jurors casting a vote for the death penalty, and officials marking the fate of the dead on lists and monuments, used theta as an abbreviation for the Greek thanatos, meaning death, a use ancient grammarians and later writers such as Persius and Isidore of Seville called the nota theta, the mark of death, and treated as proverbial for an ill omen carried by a single letter.
In modern mathematics and computer science, theta carries a much lighter, purely technical role: it is the conventional symbol for an unknown or variable angle in trigonometry and geometry, and, in the notation computer scientists use to describe an algorithm's efficiency, big theta denotes a tight bound on how its running time grows, a convention formalised in the 1970s alongside the related big O notation.
Astronomers, following the Bayer star-naming system of 1603, also use Theta to mark a constellation's eighth-brightest star, continuing the same ranked sequence used for every letter of the Greek alphabet. The gap between theta's grim ancient courtroom use and its neutral modern role as an algebra variable is a useful reminder that a letter's later technical adoption rarely carries forward any of the weight it once held.