Additives are substances that are added in a polymer to modify some of its properties. Some of the most common ones are plasticizers which are used to lower the glass transition temperature of the polymer (make it softer in a way), fillers to make it cheaper (usually chalk), oily components to improve its rheology. Others are used for electrical stability reasons or for giving the polymer a certain ability (e.g. photoinitiators for photolithography).

Of course the material itself (especially when you add inorganic compounds) is no longer a polymer but a composite material with a polymeric matrix and behaves like one.

In agreement to the previous commentator, plastics additives are all kinds of low molecular or polymeric, inorganic or organic substances that are added to a polymeric compound to render them more useful or to provide them with specific porperties.

Most polymers when synthesized, already contain catalyst residues (sometimes with inorganic supports) and usually they contain a certain minimum amount of thermal stabilizers necessary for the ganulation (at least in thermoplastics). Further thermal and oxidation stabilizers may be added afterwards and there is a host of other compounds such as UV stabilizers, chelating agents, secondary stabilizers, plasticizers, antistatic additives, impact modifiers that modify the properties of the plastic compound. I also agree that filled and reinfroced polymers with solid heterophase additives should be considered as a separate class (composites), and these "additives" should rather called fillers. The bordelines, are, however not sharp, as incompatibl blends may be considered organic filled polymers, and nano-fillerd added in few% may be considered additives.