Theta

Theta is one of the options Greeks — it measures how much value an option loses per day purely from time passing, with everything else held constant. Think of it as the daily "rent" an option pays just for existing. Decay isn't steady — it's slow with weeks or months left and accelerates sharply in the final days before expiration. Theta works against an option buyer, who loses value every day the stock doesn't move their way, and in favor of the seller, who collects that decay as time passes. It describes only the time-decay effect in isolation — it doesn't account for the stock actually moving, which can offset or overwhelm several days of decay in a single session.