A Multiplicity Fraction is the fraction of a set of star systems that have multiple stars. For example, the Milky Way would have a multiplicity fraction, as would th set of all G-Type Stars.
If a set of twenty stars is comprised of four Binary Stars plus twelve individual stars, then the set's multiplicity fraction would be 25% since four of sixteen systems have multiple stars. Yet eight of twenty or 40% of the set's stars are in multiple star systems.
The term Planet Multiplicity can be applied to Extra-Solar Planet systems in an analogous manner. Among other things, this has been used in statistical studies to verify planet discoveries, i.e., Verification by Multiplicity.