An unlisted video is a YouTube video that does not show up in searches, on the channel page or in recommendations, but that anyone who receives the direct link can watch. It sits between public (visible to everyone and discoverable on YouTube) and private (only the accounts the owner invites one by one can watch it).
In ōrbita
When you publish a YouTube video you choose between public, unlisted or private as its visibility. Details in the YouTube guide.
Also known as: unlisted, unlisted visibility, video not listed.