Full Idea
The reason why other things are called after the forms is that they participate in the forms.
Gist of Idea
Other things are named after the Forms because they participate in them
Source
Plato (Phaedo [c.374 BCE], 102a)
Book Reference
Plato: 'The Last Days of Socrates', ed/tr. Tredennick,Hugh [Penguin 1969], p.161