There could be a technical definition of an ad:
"user-visible text or image(s) meant to advertise a product or service, separate from the primary purpose or offer of the page in which it is embedded"
... or something. IANAL.
"user-visible text or image(s) meant to advertise a product or service, separate from the primary purpose or offer of the page in which it is embedded"
... or something. IANAL.
This won't really hit people doing self-serve or custom ads much, but it'll cover the vast majority of the web.