"Apparent Power" is the product of the rms voltage $U$ between the terminals of a two-terminal element or two-terminal circuit and the rms electric current I in the element or circuit. Under sinusoidal conditions, the apparent power is the modulus of the complex power.
Applicable units are those of quantitykind:ApparentPower
http://www.iso.org/iso/home/store/catalogue_tc/catalogue_detail.htm?csnumber=31891
http://www.electropedia.org/iev/iev.nsf/display?openform&ievref=131-11-41
https://cdd.iec.ch/cdd/iec61987/iec61987.nsf/ListsOfUnitsAllVersions/0112-2---62720%23UAD011
$\left | \underline{S} \right |$
$\left | \underline{S} \right | = UI$, where $U$ is rms value of voltage and $I$ is rms value of electric current.