A person or thing ready to use as a substitute or backup if needed; standing ready for emergency use; a reliable choice.
From 'stand' (to be ready) + 'by' (near, at hand). Emerged in English around the 1500s meaning something kept in reserve, and became especially common with airline terminology in the 20th century.
A standby is someone or something literally 'standing by'—waiting right there ready to go at a moment's notice, which is why standby plane tickets are cheaper (you have to be ready to leave immediately with no notice).
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