variability
Software variability refers to the ability of a software sysem or artefact to be efficiently extended, changed, customised or configured for use in a particular context [Svahnberg et al 2005].
Product line variability describes the variation (differences) between the systems that belong to a product line in terms of properties and qualities (like features that are provided or requirements that are fulfilled. [Coplien et al. 1998, Kang et al. 2002, Phol et al. 2005]