Theta is arguably the most important sensitivity of the Greeks and is certainly on a par with delta.
The characteristic of option prices to change purely as a result of the passage of time is known as Time Decay. Theta is a measure of how Time Decay affects the option premium. As such, theta is nearly always negative for bought options. This makes sense because Time Decay erodes the option value as time to expiration diminishes.
 Theta can be positive for Deep In the Money puts in certain scenarios.
Theta starts with a T and stands for Time.
Example 6.3.1. Time Decay
You pay me $1.00 for an OTM option with 10 days until expiration.
With each day that passes, let’s say the option loses $0.10 of Time Value. (Please note ...