Motion to Vacate the Chair. A "motion to vacate" is a formal proposal, either to 'vacate' (or reverse) the decision in a matter which had previously been formally ruled upon or decided, or to replace the holder of a presiding position.
In a parliamentary context, as used in a legislative body (e.g., Congress), a motion to vacate is made by a member of the body to propose that the presiding officer (or 'chair') step down (e.g., a vote of no confidence). Under the rules of the United States House of Representatives, it is considered a "privileged motion" (only when offered by direction of a party caucus or conference), where any Member can offer such a motion at any time and is subject to an immediate vote.
(See RONR Newly Revised (12th Edition), 25:1 "Suspend the Rules")