ACT/365 fixed
From ACT Wiki
A day count convention which calculates actual days in an interest period, over a 365-day conventional year, regardless of the number of days in the year.
In other words, the fraction used for interest calculations is always:
Actual days / 365
This applies in leap years as well as normal years.
ACT/365 fixed is the convention most often used in the short term GBP market.