A put option is a financial instrument that gives the holder the right, but not the obligation, to sell the underlying asset at a predetermined price, known as the strike price. The holder of a put option has the right to exercise the option and sell the underlying asset at the strike price, but is not required to do so.

Here are some key characteristics of put options:

1. Right to sell: The holder has the right to sell the underlying asset.

2. Strike price: The predetermined price at which the holder can sell the underlying asset.

3. Expiration date: The last day on which the option can be exercised.

4. Premium: The price paid by the holder to acquire the option.

5. Underlying asset: The security or asset that the option is based on.

Put options are often used for various purposes, including:

1. Speculation: To bet on price decreases in the underlying asset.

2. Hedging: To lock in a sale price for an asset already owned.

3. Income generation: To sell options and collect premiums.

4. Spreads: To combine options with different strike prices or expiration dates.

5. Protection: To limit potential losses by locking in a sale price.

Remember, put options give the holder the right to sell, but not the obligation, so the holder can choose to exercise the option or let it expire worthless.