theta

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The eighth letter of the Greek alphabet.

from The Century Dictionary.

• noun As used in formulæ of mathematics and engineering, theta (θ) usually denotes:
• noun the entropy, or thermodynamic function or factor by which the absolute temperature is to be multiplied to indicate an amount of heat energy.
• noun 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.
• noun A letter of the Greek alphabet corresponding to the English th in thin, etc.
• noun 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.
• noun A function which occurs in probabilities, and is expressed by the integral fe—tdt.

from the GNU version of the Collaborative International Dictionary of English.

• noun 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.
• noun (Math.) one of a group of functions used in developing the properties of elliptic functions.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

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

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the 8th letter of the Greek alphabet