Theta, or time decay, explained simply — why options lose value every day, why it hurts buyers, and why sellers love it.
Theta is the Greek that measures time decay — how much value an option loses each day, just because time is passing.
An option is partly a bet on time. The more time left to expiry, the more chance the market has to move your way, so the option costs more. As expiry approaches, that chance shrinks, and the option’s time value steadily drains away. That daily loss is Theta.
If you buy an option and the market does nothing, you still lose a little every day. This is why so many buyers are puzzled when they were "right" but still lost — time quietly worked against them.
The seller is on the other side. Every day the option loses time value, the seller is closer to keeping the premium. This is why sellers often say "Theta is the seller’s best friend."
Time decay is not linear. It is slow when expiry is far away and accelerates sharply in the final days, especially for at-the-money options. This is why the last few days before expiry can be brutal for buyers and rewarding (but risky) for sellers.
We teach Theta and all the Greeks with live examples in both the Option Buying and Options Selling courses.