From Longman Dictionary of Contemporary Englishon standbyon standbya) READYready to help immediately if you are needed A special team of police were kept on standby. b) if you are on standby to do something, for example to travel by plane, you are on a list of people who may be allowed to do it if places become available We can put you on standby. → stand by → standby
Examples from the Corpus
on standby• City firefighters have been on standby for the past three days.• Other airlines said they would accept the passengers on standby. 