Time Decay

See: Theta Decay.

It sounds like what happens when you don't floss, but it actually refers to the way that the time value of a stock option decays or declines with time.

The closer to the expiration date, the smaller the time value of the option. (Time value: as in...if you had an option to buy KO for $50 when KO is today trading at $42, and that option expired in 2 weeks, it probably wouldn't be worth much, because the odds of KO trading much above $50 in the next 2 weeks is relatively low. But if you had Theta (or time) of 3 years before the option expired, then yeah, that option could be worth a ton.)

Example:

You sold puts on GOOG at $450 for $35 which expire in 4 months. The stock today is at $600. That is, you sold the right for someone to make you buy shares of GOOG at $450 any time between now and 4 months from now for 35 bucks. Things go along and, well, GOOG just stays pretty flat, doing a whole lot of nothing.

It's now 3 days before those put options expire (we've gone 3.9 months with a whole lot of nothing happening in GOOG). The stock is still around $600 a share. What are the odds it plummets $150+ in 3 days? Really low. So the value of those puts is almost fully expired—its Theta has decayed to just 3 days' worth of trading time, and it is highly likely you just collect your 35 bucks, walk away, and buy yourself a really nice burger at a Manhattan eatery.