Conclave, appropriately enough, comes from the Latin word meaning “a room or apartment that can be locked up.” Conclave is formed from com (meaning “with, together, jointly”) plus clavis (meaning “key”). The very earliest use of conclave was to name a private chamber or closet, but that sense disappeared by the mid-1700's.

In the Roman Catholic church, conclave names the assembly of cardinals gathered to elect a new pope, and it names the system of strict seclusion to which they are submitted. From 1059 on, the election became the responsibility of the cardinals. When, after the death of Clement IV in 1268, the cardinals dithered for two years, the local magistrate locked them in the episcopal palace and fed them only bread and water until they elected Gregory X. The system of meeting in closed conclave was codified in 1904 by Pius X. Today, election requires a majority of two-thirds plus one (unless, according to rules established by John Paul II in 1996, such consensus does not occur within a specified number of ballots; then the cardinals may agree to an absolute majority). Voting by secret ballot takes place twice a day; ballots are burned after each vote. Until the needed majority is obtained, the ballots are burned with wet straw to make black smoke; white smoke issuing from the Vatican Palace indicates that the vote has resulted in the election of a new pope.

From Merriam-Webster's Collegiate Encyclopedia and Webster's Third New International Dictionary, Unabridged.