This us the risk of an exchange rate changing between the transaction date and the subsequent settlement date, i.e. it is the gain or loss arising on conversion.
This type of risk is primarily associated with imports and exports. If a company exports goods on credit then it has a figure for debtors in its accounts. The amount it will finally receive depends on the foreign exchange movement from the transaction date to the settlement date.
As transaction risk has a potential impact on the cash flows of a company, most companies choose to hedge against such exposure. Measuring and monitoring transaction risk is normally an important component of treasury risk management.