Both alternating current and direct current are measured in amperes. But how is the ampere defined for an alternating current?

(N/A) For a direct current $(DC)$,$1$ ampere is defined as $1$ coulomb of charge flowing per second.
An alternating current $(AC)$ changes its direction periodically with the source frequency. If we were to measure the average current,it would be zero over a full cycle,which is not useful for power measurement.
Therefore,the $AC$ ampere is defined based on a property that is independent of the direction of current,which is the heating effect (Joule's heating).
$1$ ampere of $AC$ is defined as the value of alternating current that produces the same amount of heat in a given resistor as $1$ ampere of $DC$ would produce in the same resistor over the same time interval. This is known as the root-mean-square $(RMS)$ value of the current.