Definition of 'Flat'
1. A price that is neither rising nor declining.
2. In forex, the condition of being neither long nor short in a particular currency. Also referred to as 'being square'.
3. A bond that is trading without accrued interest.
Investopedia explains 'Flat'
1. If a stock over the last month has been trading around $30, it can be thought of as trading flat.
2. If you had no positions in the U.S. dollar or your long and short positions canceled each other out, you would be flat or have a flat book.
3. A bond is trading flat if the buyer of the bond is not responsible for paying the interest that has accrued since the last payment (accrued interest is usually part of the bond purchase price). Bonds that are in default trade flat.