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 the 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.