A day count convention which calculates actual days in a time period, over the actual number of days in a year.
In a normal year, this will be 365 days.
In a leap year it is 366 days.
The ACT/ACT convention is also sometimes known as ACT/365, because the denominator is usually 365.
Contrasted with ACT/365 fixed, where the denominator is always 365, including in a leap year.