What is "nano" in the context of a filled dielectric?

Some evidence in the literature suggests that nano fillers in a polymer matrix can provide beneficial dielectric properties. The obvious question is "What is `nano'?" Where and based on what criteria is a system nanofilled or microfilled? A statistical approach is applied to this problem to develop criteria based on worst case field conditions over critical distances. The criteria are based on previous work related to electrical tree initiation and insight gained from large scale 3-D field simulations of filled systems