Using an importance function describing the capability of a system for producing fission neutrons, a new definition of the subcriticality is proposed, which has the physical meaning of a multiplication factor in a real subcritical system with external sources. This multiplication factor ks, which expresses the number of fission neutrons produced by a fission neutron in a steady state, is different from the usual criticality factor or the effective multiplication factor keff, since the former is calculated from the inhomogeneous equation with external source, whereas the latter is calculated from the homogeneous criticality equation without external source.