What Is the Practical Difference between Theta and the Implied Volatility of an Option?

Theta is a measure of the rate of time decay, representing the loss in an option's value per day. Implied Volatility (IV) is an input into the option pricing model, representing the market's expectation of the underlying asset's future price fluctuations.

High IV increases an option's price, but that higher price will decay faster (higher Theta). Theta is a derivative of the price, while IV is an input.