A mismatch in the timing at which interest rate assets and liabilities are repriced.
A positive gap (assets repricing more quickly than liabilities) means an exposure to falling interest rates and vice versa.
Banks and other financial institutions commonly have a 'structural' interest gap, resulting from the nature of their business and the structure of their balance sheets.
This structural interest gap is usually negative.
The negative interest gap results from shorter-term liabilities funding longer term assets.