The endowment is the central ordinance of the Latter-day Saint temple, and it is also the point at which a reference work has to decide what it is willing to print. The Church of Jesus Christ of Latter-day Saints holds the detail of the ordinance sacred and asks that it not be discussed outside the temple. Transcripts and recordings circulate, obtained without the church's consent, and any reader determined to find them will. The atlas does not reproduce them, and this article says so rather than leaving the omission to look like ignorance.
What can be said, because the church says it, is this. The endowment is received once for oneself, ordinarily before a mission or a marriage, and afterward may be received many times by proxy on behalf of people who have died. The church describes it as a course of instruction in the plan of salvation, presented in sequence, in which the participant makes covenants with God and receives what the movement calls an endowment of power. It takes place inside a dedicated temple, not in an ordinary meetinghouse, and lasts a little under two hours. Participants dress in white. Admission requires a current temple recommend, issued after interviews with a bishop and a stake president, so a parent, a sibling or a friend outside the church cannot attend, and neither can a member without a recommend.
Those who have received the endowment wear the temple garment as ordinary underclothing afterward. The church has described it publicly as a reminder of the covenants made, and that is as far as this entry goes.