standby

Từ điển Oxford Advanced Learner 8th

stand·by [standby standbys] noun, adjective BrE [ˈstændbaɪ] NAmE [ˈstændbaɪ]
noun (pl. stand·bys)
a person or thing that can always be used if needed, for example if sb/sth else is not available or if there is an emergency
•I always keep a pizza in the freezer as a standby.
•a standby electricity generator
•Wilson has been called into the squad as a standby.
•A dark suit is a useful standby for any formal occasion.
Idiom: ↑on standby
 
adjective only before noun
a standby ticket for a flight, concert, etc. cannot be bought in advance and is only available a very short time before the plane leaves or the performance starts