actual/360
Actual/360 is an interest-accrual method in which the applicable annual rate is applied to the outstanding principal for the number of days that have passed, with the resulting time fraction calculated using 360 as the annual base. It is commonly used for certain short-term lending and central-bank transactions, including Eurosystem monetary policy operations.
Source: European Central Bank (ECB) (source). This glossary entry is an independently worded adaptation of the cited information.