Platonism

Concise Oxford English Dictionary © 2008 Oxford University Press:
Platonism /ˈpleɪt(ə)nɪz(ə)m/

▶noun the philosophy of Plato or his followers, especially that relating to Plato's theory of ‘ideas’ or ‘forms’, in which abstract entities (‘universals’) are contrasted with their objects (‘particulars’) in the material world.

■ the theory that numbers or other abstract objects are objective, timeless entities, independent of the physical world and of the symbols used to represent them.

– derivatives
Platonist noun & adjective.