The belief that though abstract terms (i.e. universals) do exist in thought or as names, corresponding abstract objects do not exist in reality.
For example, though we can conceive of a universal that we call the horse, this mental entity does not exist outside of our minds — in the real world there are only individual horses.
1. According to IF Stone, Antisthenes was propably the first person to raise this criticism of the Platonic Forms.