Theta

The rate at which an option loses value as time passes.

Theta quantifies time decay, the amount an option premium erodes each day, all else equal. It works against option buyers and in favor of sellers. Theta accelerates as expiration nears, especially for at-the-money options, which is why short-dated options decay fastest.

As an options seller, he welcomed theta, pocketing a little value each day the stock stood still.