something that can be relied on when needed
"The backup generator serves as a reliable standby during power outages."
an actor able to replace a regular performer when required
"The understudy was on standby to take over if the lead actress fell ill during the performance."
A state of readiness without immediate involvement; remaining in preparation for (a sudden or unforeseen event or situation).
"The pilot kept the engine on standby, ready to take off at a moment's notice."
In plain English: A standby is someone who waits to take over if everyone else fails or gets sick.
"Keep your phone on standby in case I need to contact you later."
Usage: Use "standby" to describe a person or system kept ready for action when needed. Do not confuse this noun with the verb phrase "stand by," which means to support someone or wait patiently without intervening immediately.
To wait briefly, as for additional communication by radio or telephone; alternative spelling of stand by
"The pilot waited on standby for further instructions from air traffic control before landing."
In plain English: To standby means to wait ready for action without doing anything yet until you are told to start.
"The generator was on standby in case the power went out again."
The word standby comes directly from the phrase stand by, which originally meant to remain nearby or ready to act when needed. It entered English as a single noun describing someone or something kept on call for immediate use.