The option's theta is a measurement of the option's time decay. The theta measures the rate at which options lose their value, specifically the time value, as the expiration date draws nearer. Generally expressed as a negative number, the theta of an option reflects the amount by which the option's value will decrease every day.
A call option with a current price of $2 and a theta of -0.05 will experience a drop in price of $0.05 per day. So in two days' time, the price of the option should fall to $1.90.
Longer term options have theta of almost 0 as they do not lose value on a daily basis. Theta is higher for shorter term options, especially at-the-money options. This is pretty obvious as such options have the highest time value and thus have more premium to lose each day.
Conversely, theta goes up dramatically as options near expiration as time decay is at its greatest during that period.
The chart above illustrates the relationship between the option's theta and the volatility of the underlying security which is trading at $50 a share and have 3 months remaining to expiration.
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