The Actual Cost (AC) is a function of the time and it is a measured of how much money has been spent in a project. It's measured in units of economic value ($). Mathematically, the domain of the Actual Cost (AC) is time and the range is economic value ($). The Actual Cost (AC) is clearly associated with the Earned Value (EV) because the units of work that are measured by AC, are the same that EV has added as progress. In other words, when a unit of work has been done, EV adds the planned value of cost for that unit of work and AC adds the real cost  for that unit of work.