Full Idea
Socrates did not regard the universals or the objects of definitions as separate existents, while Plato did separate them, and called this sort of entity ideas/forms.
Gist of Idea
Socrates did not consider universals or definitions as having separate existence, but Plato made Forms of them
Source
report of Socrates (reports of career [c.420 BCE]) by Aristotle - Metaphysics 1078b30
Book Reference
Vlastos,Gregory: 'Socrates: Ironist and Moral Philosopher' [CUP 1992], p.92