"Above the fold" and ads

Dear Google has modified their ranking algorithm (again) to punish pages that have too high an ad to content ratio "above the fold":
"If you click on a website and the part of the website you see first either doesn’t have a lot of visible content above-the-fold or dedicates a large fraction of the site’s initial screen real estate to ads, that’s not a very good user experience. Such sites may not rank as highly going forward.

We understand that placing ads above-the-fold is quite common for many websites; these ads often perform well and help publishers monetize online content. This algorithmic change does not affect sites who place ads above-the-fold to a normal degree, but affects sites that go much further to load the top of the page with ads to an excessive degree or that make it hard to find the actual original content on the page."
Source.

And ghacks wants to know "The definition of what the company considers to be an ad"