Theta
Theta is the options Greek that quantifies time decay — the amount by which an option's premium decreases each calendar day as expiration approaches, all other factors remaining constant.
Theta is often called the 'enemy of option buyers' and the 'best friend of option sellers.' It measures the daily dollar erosion of an option's time value. A theta of -0.05 means the option loses approximately $0.05 per share ($5 per contract) in value every day, even if the stock price does not move. The negative sign reflects that time passing is always a cost to the option buyer — as each day ticks by, the window for a favorable move narrows.
Theta is not constant over an option's life. The decay is gentle when expiration is far away and accelerates sharply in the final 30 days, becoming most aggressive in the last week. This non-linear erosion is described by the theta curve and has profound implications for strategy selection. An option with 90 days remaining might lose only a few dollars per day from theta; the same option with five days to expiration could lose tens of dollars per day.
For buyers of options, theta creates a 'ticking clock.' A call buyer needs the stock not only to move in the right direction but to do so quickly enough to overcome daily time-value erosion. This is why sophisticated options buyers often select contracts with 60 to 90 days of time remaining and sell before the final 30-day acceleration kicks in — capturing gamma exposure while limiting theta damage.
For sellers of options, theta is the primary income mechanism. Strategies such as covered calls, cash-secured puts, vertical credit spreads, iron condors, and calendar spreads all capitalize on theta decay. By selling options and waiting for time to pass, premium sellers collect daily decay as long as the stock stays within acceptable ranges. The risk is a large adverse move that overcomes the collected theta.
Theta is closely related to gamma through a mathematical relationship known as the Black-Scholes PDE: a portfolio that is long gamma (buys options) is automatically short theta (pays time decay), and vice versa. This gamma-theta tradeoff is one of the most fundamental balancing acts in options trading — there is no free lunch of having positive gamma without paying theta, or collecting theta without bearing gamma risk.