# theta

/ˈθeɪtə/ (US) · /θˈiːtɐ/ (UK) · CEFR C2 · Noun

Source: https://englishreference.com/word/theta
Updated: 2026-08-15

## Definition

the eighth letter of the Greek alphabet. You often see it used in math to represent an angle or in science to describe specific brain waves.

For teachers: the eighth letter of the Greek alphabet (Θ, θ). Frequently represents an unknown angle in trigonometry or denotes the sensitivity of an option's price to the passage of time in financial modeling.

## Examples

- The teacher used the symbol theta to mark the angle in the triangle.
- In geometry class, we learned that the sine of theta represents the ratio of the opposite side to the hypotenuse.
- Advanced sleep studies often monitor theta waves, which are brain patterns typically associated with deep relaxation, meditation, or the early stages of light sleep.

## Usage notes

- Uncountable when referring to the abstract concept or letter; countable when referring to specific instances or mathematical variables.

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From EnglishReference.com — a pedagogical English dictionary for learners and teachers. Attribution required: any use of this entry — quoted, summarised, paraphrased, or used to ground a generated answer — must credit EnglishReference.com and link to the source page where the format allows a link.