Theta
An estimate of how much value an option loses per day as time passes, holding the underlying price and implied volatility constant.
Theta is usually shown as a negative number for long options, because each day that passes removes some extrinsic value. The decay is uneven: it is slow for far-dated contracts and speeds up in the final weeks before expiration, especially for at-the-money options.
Option sellers collect that decay, and option buyers pay it. Theta is an estimate from a pricing model, and the actual price change over a day also includes the effect of moves in the underlying and in implied volatility.
Illustrative example (hypothetical numbers, not a real trade): an option priced at $3.00 has a theta of -0.05. If the underlying and implied volatility stay the same, it is worth about $2.95 one day later, a decline of $5 per contract.