A type of binary option that provides a fixed payout if the underlying asset reaches or “touches” a predetermined price level (barrier) at least once during the life of the contract. Unlike standard options, it does not require the asset to remain above or below the barrier, only that it touches it at any point before expiration. 

This option is typically used in directional trading strategies, where the investor believes the asset will move decisively in a specific direction and reach a particular level within a set timeframe. It is commonly traded in foreign exchange, commodities, and other derivatives markets. 

For example, if a trader buys a one touch option on GBP/USD with a barrier of 1.3000 and the exchange rate rises to 1.3000 before expiration, the option pays out the full agreed amount, even if the price later falls below the barrier. 

One touch options are attractive because they offer potentially high payouts in return for a relatively simple price condition. However, they are also sensitive to volatility, time to maturity, and how close the current price is to the barrier. They are often used alongside no touch options or other exotic derivatives to construct customized risk or investment profiles.