Theta (Θ, θ) is the eighth letter of the Greek alphabet and is commonly used in various fields, including mathematics, physics, and engineering. In mathematics, it often represents an angle in geometry or trigonometry, while in physics, it can denote temperature or a specific variable in equations. In finance, theta is a measure of the rate at which an option's price decreases as it approaches its expiration date, known as time decay. This concept is crucial for traders and investors who use options, as it helps them understand how time affects the value of their investments.