a. [ Gr. &unr_; a perpendicular line. See Cathetus. ] (Math.) Making a right angle; perpendicular, as two lines or two sides of a triangle, which include a right angle. [ 1913 Webster ]

n. [ L., fr. Gr. qh^ta, the Greek letter θ, Θ. ] A letter of the Greek alphabet corresponding to th in English; -- sometimes called the unlucky letter, from being used by the judges on their ballots in passing condemnation on a prisoner, it being the first letter of the Greek qa`natos, death. [ 1913 Webster ]

Theta function (Math.), one of a group of functions used in developing the properties of elliptic functions. [ 1913 Webster ]

‖n. pl. [ NL., fr. E. type + Gr. &unr_; to put, set. ] Printers; -- used in the name of an association of the master printers of the United States and Canada, called The United Typothetae of America. [ 1913 Webster ]