standby

Words with a similar base: standby

'standby'

[standby.n.01] as in: something that can be relied on when needed

'standby'

[understudy.n.01] as in: an actor able to replace a regular performer when required

'standby'

[standby.s.01] as in: ready for emergency use

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Examples:
a standby generator
a standby crew