A Greek that measures how much value an option loses each day purely due to the passage of time.
Theta quantifies time decay — the rate at which an option loses value each day as it approaches expiry, all else being equal. Theta is negative for option buyers, meaning long options lose value daily, and positive for option sellers, who benefit from that same decay. Theta decay accelerates as expiry nears, which is why many premium-selling strategies are structured around the final days before an option’s expiry.
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