A day-count convention determines the portion of a year represented by an interest-bearing period. It combines a method for counting elapsed days with a specified year-length basis, such as 360 or 365 days. The resulting fraction affects the interest charged or paid on financial instruments, including loans, deposits, and bonds. For Eurosystem monetary policy transactions, the applicable basis is actual/360: the real number of days is divided by 360.
Source: European Central Bank (ECB) (source). This glossary entry is an independently worded adaptation of the cited information.