standby

nouns

something that can be relied on when needed

standby

is derived from

is a type of (narrower)

an artifact

"how does this thing work?"

an actor able to replace a regular performer when required

standby, understudy

is derived from

ready for emergency use

"a standby generator"; "a standby crew"

standby

is similar to

being of second rank or importance or value; not direct or immediate

"the stone will be hauled to a secondary crusher"; "a secondary source"; "a secondary issue"; "secondary streams"