Theta (Θ, θ) Definition

Theta (Θ, θ) is the eighth letter of the Greek alphabet, derived from the Phoenician letter Teth. In the system of Greek numerals it has the value 9.

Usage in Mathematics & Science

The uppercase Theta (Θ) is used for:

Dimension of temperature, by SI standard (in italics).
Dimensionless temperature in transport phenomena.
An asymptotically tight bound in the analysis of algorithms (big O notation).
A certain ordinal number in set theory.
Pentaquarks, exotic baryons in particle physics.
A brain signal frequency (beta, alpha, theta, delta) ranging from 4–8 Hz.
One of the variables known as Greeks in finance, representing time decay of options or the change in the intrinsic value of an option divided by the number of days until the option expires.
A variable indicating temperature difference in heat transfer.
Measuring the angle of incident X-ray beam during XRD.

The lowercase Theta (θ) is used for:

A plane angle in geometry.
An unknown variable in trigonometry.
A special function of several complex variables.
One of the Chebyshev functions in prime number theory.
The potential temperature in meteorology.
The score of a test taker in item response theory.
Theta Type Replication: a type of bacterial DNA replication specific to circular chromosomes.
Threshold value of an artificial neuron.
A Bayer designation letter applied to a star in a constellation; usually the eighth star so labelled but not necessarily the eighth-brightest as viewed from Earth.
The statistical parameter frequently used in writing the likelihood function.
The Watterson estimator for the population mutation rate in population genetics.
Indicates a minimum optimum integration level determined by the intersection of GG and LL schedules (The GG-LL schedules are tools used in analyzing the potential benefits of a country pegging their domestic currency to a foreign currency.)
The reserve ratio of banks in economic models.
The ordinal collapsing function developed by Solomon Feferman.
Heaviside step function.
In pharmacology, the fraction of ligand bound to a macromolecule based on the Hill Equation.

The letters of the Ancient Greek Alphabet, which are frequently utilized in math and science:

“Theta.” Wikipedia, Wikimedia Foundation, 8 Apr. 2020, en.wikipedia.org/wiki/Theta.