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

For a direct current ($DC$) $1$ ampere $=\frac{1 \text { Coloumb }}{\mathrm{S}}$

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.