The Depression-era charity mission in New York City run by Edith Keeler, where Kirk and Spock found work and shelter while searching for Doctor McCoy in 1930.
The Twenty-First Street Mission is the setting of much of the Original Series episode The City on the Edge of Forever.
After passing through the Guardian of Forever, Kirk and Spock arrive in New York in 1930 and take refuge in the mission's basement. Its director, Edith Keeler, gives them odd jobs and a room, and speaks to the men at her soup kitchen about a hopeful future among the stars.
McCoy later stumbles into the same mission and is nursed there by Keeler. Spock learns that her survival would lead to a pacifist movement delaying American entry into the Second World War, and Kirk has to let her die in a street accident outside.