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A measure of change in the value of an option compared with the change in the time to expiry. Also known as time decay or time-value decay. Option values decline as their expiry date approaches. As an option that is out of the money moves closer to its expiry date the likelihood that it will become in the money declines, reducing its value. If it is deeply in the money it is likely to be more expensive than other options with different strike prices, which will also reduce its value.
See also: Delta, Gamma, Rho, Vega