Theta (θ) is a letter from the Greek alphabet, commonly used in various fields such as mathematics, physics, and engineering. In mathematics, it often represents an angle in trigonometry, where it helps describe the relationships between the sides and angles of triangles. In finance, theta is a measure of the time decay of options, indicating how much the price of an option decreases as it approaches its expiration date. This concept is crucial for traders in the options market, as it helps them understand the impact of time on their investments.