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

Explanation

For a direct current (DC) 1 ampere = `"1 Coloumb"/S`
An AC current changes direction with the source frequency and the attractive force would average to zero. Thus, the AC ampere must be defined in terms of some property that is independent of the direction of current.
Joule.s heating effect is such property and hence it is used to define rms value of AC.
1 ampere current in AC means, the amount of heat produce in resistance of 1 `Omega` by DC current as same amount produced in AC is known as 1 ampere of AC current.