http://youtube-global.blogspot.com/2010/05/more-choice-for-users-unlisted-videos.html

Later today, we'll be rolling out a new choice ...: unlisted
videos.

With this feature, you can mark your videos as "unlisted." This means only
people who have the link to the video will be able to watch it. It won’t appear
in any of YouTube’s public pages, in search results, on your personal channel or
on the browse page. It’s a private video, except you don’t need a YouTube
account to watch it and there is no limit to the number of people who can view
it. You’ll get a link when you upload the video and then it’s up to you to
decide who to share it with.