Today, YouTube announced a feature I have been wanting for years: the ability to designate a video as "unlisted." Before today, we had two choices for video access: public (viewable - and searchable by anyone, and private - up to 25 other YouTube account holders. Recognizing the need for a third option, YouTube delivered the new "Unlisted" option.
"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."
Read more here.