In the wake of rapid developments in nanotechnologies and nanosciences, the need for an internationally agreed definition of a 'nanomaterial' has gained more urgency. A number of definitions are currently available. These are, however, mainly based on size parameter(s), and fall short in terms of applicability to particulate materials that only have a size fraction in the nano-scale, or that contain primary nanostructures in highly agglomerated or aggregated forms. To overcome these shortcomings, we are proposing a complementary definition based on volume specific surface area (VSSA) that can be used as a basis for classification and regulation purposes. (C) 2010 Elsevier Ltd. All rights reserved.