to nominate or select for a duty, office, purpose, etc.; appoint; assign.
adjective
5.
named or selected for an office, position, etc., but not yet installed (often used in combination following the noun it modifies): ambassador-designate.
pp. adj. from designate. Designated hitter introduced in American League baseball in 1973, soon giving wide figurative extension to designated, e.g. designated driver, by 1986.