"Power Factor", under periodic conditions, is the ratio of the absolute value of the active power $P$ to the apparent power $S$.
Applicable units are those of quantitykind:PowerFactor
http://www.iso.org/iso/home/store/catalogue_tc/catalogue_detail.htm?csnumber=31891
http://www.electropedia.org/iev/iev.nsf/display?openform&ievref=131-11-46
$\lambda = \left | P \right | / \left | S \right |$, where $P$ is active power and $S$ is apparent power.