In economics, the unit of account is a unit of measurement. The quality that it measures is market value.
Goods for sale in a market are priced using a unit of account. In this manner the value is measured by the seller and expressed to the buyer.
Contracts of credit or debt are denominated in a unit of account. In this manner the agreed value of the debt is measured, and the method of settling the debt is defined.
Whilst all market participants are free to use any unit of account that they prefer, most markets have only a few widely accepted units of account. For example in Japan the most universal unit of account is the yen.