A private or secret meeting is a conclave. What are some reasons you might hold a conclave? You might be convening the executive committee on the board of a company, or you might be planning a surprise party.
In Catholicism, a conclave is specifically a meeting of Roman Catholic cardinals who are locked in a room in seclusion when they need to choose a new pope. The word conclave comes from the Latin and originally meant "a locked room." The school's band boosters held a conclave to discuss the harsh district budget cuts and how those cuts might affect all the other music programs at school.