A traditional Vulcan test of maturity in which a child must survive alone for ten days in the desert of Vulcan's Forge without food, water or weapons.
The kahs-wan is an old Vulcan rite of passage. A child of about seven is sent into the desert known as Vulcan's Forge and must survive there for ten days without food, water or weapons. To fail once is no disgrace, but success marks the first step towards adulthood.
The ordeal is central to the animated episode Yesteryear. The young Spock, taunted for his human blood, sets out into the Forge early to prove himself, followed by his pet sehlat I-Chaya. The adult Spock, travelling back through the Guardian of Forever in the guise of a cousin named Selek, must save the boy's life.
I-Chaya is fatally wounded defending the child, and the young Spock's decision to let him die peacefully fixes his choice of the Vulcan way. T'Pol mentions her own kahs-wan in the Enterprise episode The Catwalk.