Theta - Theta is the time decay, is an estimate of how much the theoretical value of an option decreases when 1 day passes and assuming there is no movement in either the stock price or volatility. It is usually expressed in points lost per day, when all other conditions remain the same. It basically represents a loss in the option's value as time passes. A long position on option will always have a negative theta, and a short position will always have a positive theta.