The costs genuinely incurred for the work counted as earned value, up to a given date. Actual cost only means something when it covers the same scope earned value covers.
Actual cost answers what the completed work has cost, read off the ledger rather than off the schedule. Comparing it with earned value gives cost variance, which is why the two must cover exactly the same work: costs booked against work not yet credited, or credit taken for work whose invoices land next month, make the difference between them meaningless. Invoicing in arrears is the common cause, and it flatters cost variance until the bill catches up.
See also
The budgeted cost of the work actually completed at a given date. Earned value prices finished work at baseline rates, never at what the work really cost.
The budgeted cost of the work the baseline scheduled to be complete by a given date. Planned value = budget at completion × planned percent complete.
Where this comes up
Two ratios off one earned value figure. SPI divides it by the plan and reads schedule. CPI divides it by the bill and reads cost. Below 1 is bad news in both, for different reasons.
Three figures read at one date. Planned value is what the baseline said would be finished by now, earned value is what did finish, actual cost is what finishing it cost.