stand by, standby

i. In radio communications, the term means, “Wait for a response,” or, “Wait and I will call you.” It does not imply any onward automatic clearance.

ii. A passenger able to board a flight at the last moment provided a seat is available.

iii. In an air defense alert, it means be prepared to take off within the stipulated time (e.g., standby 2 min).

iv. A fallback system if the main one malfunctions (e.g., a standby hydraulic system). Also a duplicated system.

v. As it pertains to flight crew duty and rest periods, it is the period during which a crewmember may be called for duty with minimum notice.