I believe it is time until the visible area is rendered. Neither domready nor load time are very meaningful since all of the interesting rendering happens in javascript, triggered by the onready event.
The article isn't clear on it, but there have been studies (http://baymard.com/blog/making-a-slow-site-appear-fast) that show the perception of fast loading is actually more important than the real thing. Showing the user a "Loading..." graphic is the most common manifestation of this, but there are others. (Unfortunately, the original Forrester-Akamai study seems to be unavailable.)
