High-def as in a full HD 1080p resolution.
From the link:
At the NewTeeVee Live conference on Thursday, YouTube director of product management Hunter Walk announced that the video-streaming service is getting a new high-quality streaming option: full HD, or “1080p” resolution. The current “high-quality” option, when available on YouTube videos, is 720p, referring to the number of horizontal scan lines that make up the image.
Walk said the new resolution, as well as a new full-screen player, will roll out to all users within days.
YouTube co-founder Steve Chen announced high-quality YouTube viewing at NewTeeVee 2007. He also said, then, that YouTube stores all video it receives at the resolution it’s uploaded at. So when YouTube ads a resolution option, as it did then and is doing now, it simply needs to re-encode videos for the new player, not get new raw content.