[a. Gr. θῆτα: see def.] The eighth letter of the Greek alphabet, Θ, θ (see TH).
In ancient Greece, on the ballots used in voting upon a sentence of life or death, θ stood for θάνατος, death; hence in allusive use.
1603. Daniel, Def. Ryme, H iv. Setting his Theta or marke of condemnation vppon them.
1616–61. Holyday, Persius, iv. 317. And the black theta, signe of deadly shame, Thou can’st prefix ’fore an offenders name.
1682. Sir T. Browne, Chr. Mor., I. § 22. At the Tribunal … wherein iniquities have their natural Theta’s, and no nocent is absolved by the verdict of himself.
1789. M. Madan, trans. Persius (1795), 103. Able to fix the black theta to vice.
b. attrib. and Comb., as theta-sounding adj.; theta-function, in Math., a name for two different functions: (a) the sum of a series from n = – ∞ to n = + ∞ of terms denoted by exp(n–2a + 2na); also extended to a similar function of several variables; (b) a function occurring in probabilities, expressed by the integral ∫e–2dt; theta-phi diagram, the temperature-entropy diagram, which represents the heat-units converted into work per pound of working fluid (θ = absolute temperature, and φ = entropy).
1871. M. Collins, Mrq. & Merch., III. iii. 88. You [English] are a theta-sounding people.
1879. Cayley, Coll. Math. Papers, X. 475. We have thus an addition-with-subtraction theorem for the double theta-functions.
1901. Pract. Engineer Pocket Bk., 166. The temperature-entropy diagram is usually called the θφ (theta-phi) diagram.
