theta

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. The eighth letter of the Greek alphabet. See Table at alphabet.

Century Dictionary and Cyclopedia

1. n. As used in formulæ of mathematics and engineering, theta (θ) usually denotes:
2. n. the entropy, or thermodynamic function or factor by which the absolute temperature is to be multiplied to indicate an amount of heat energy.
3. n. The angle, in angular measure, or at the center, through which a radius vector has swept from a zero position, or through which a line has swept in a rotation.
4. n. A letter of the Greek alphabet corresponding to the English th in thin, etc. It was sometimes called the unlucky letter, because it was used by the judges in passing condemnation on a prisoner, it being the first letter of the Greek θάνατος, death.
5. n. A sort of complication of an exponential function, being expressed by a series from n =—∞ to n = + ∞ of terms the logarithm of each of which is na + 2 na. A theta function of several variables, x₁, x₂, … x_n, is Σ exp. (φ + Σmx_n x_n), where φ is a quadratic function of the constants m₁, m₂, … m_n.
6. n. A function which occurs in probabilities, and is expressed by the integral fe—tdt.

Wiktionary

1. n. the name for the eighth letter οf the Modern Greek alphabet, ninth in Old Greek.
2. n. mathematics The measure of an angle.

GNU Webster's 1913

1. n. 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.

WordNet 3.0

1. n. the 8th letter of the Greek alphabet