The Shinigami Eye Deal, or the Shinigami Eye Trade, is a deal made between a human and a Shinigami.
The Shinigami Eye deal allows a Shinigami to grant the human it is currently possessing Shinigami Eyes so that they can see the name and lifespans of other humans. In exchange for this power, the human gives the Shinigami half of their remaining lifespan.
If a human makes the Eye Trade and then loses possession of their Death Note, they will also lose their Shinigami Eyes. If the human regains ownership of the Death Note, they will not regain their Shinigami Eyes. If they wish to regain the Shinigami Eyes, they must halve their lifespan again. The process can be repeated every time they lose and then regain ownership of the Death Note.
Characters who made the Shinigami Eye Deal: