Theta: Eighth letter of the Greek alphabet
Theta (uppercase Θ / lowercase θ), is a letter in the Greek alphabet.
It represents the "th" sound (as in "think") in Ancient and Modern Greek. In the system of Greek numerals, it has a value of 9. The Cyrillic letter Ѳ came from theta.
In mathematics, the lowercase θ is used as a variable to represent an angle, and the uppercase Θ is used in big-theta notation (a variant of big-O notation). The lowercase θ is also used to represent the potential temperature in meteorology.
References
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