Depends on what you mean by "definition". They say[0] they only accept ads that do not disrupt the reading flow, can be distinguished from the rest of the content, and has a reasonable size (all clearly defined). (They also have more criterias, but those are the main ones).

But the problem is, that is very poorly enforced in practice. Whenever I use ABP, I see ads breaking the standard everywhere. To the point where I can only assume the Acceptable Ads definition is just a deflection.

And of course, there's the whole "you need to pay to get on the list" which creates some very awful incentives.

[0]: https://acceptableads.com/standard/