person on standby; person on a waiting list; person waiting
General term for someone who is waiting for their turn, such as on a waiting list, in a queue, or on standby for a service or event.
Popular restaurants always have many people on the waiting list.
Since there was a cancellation, I contacted the person on standby.
person in quarantine; person isolating
Used specifically for someone who is quarantining or isolating, often in medical or public health contexts. This usage became more common during the COVID-19 pandemic.
Close contacts were asked to stay home as persons in quarantine.
待ち人 is a more poetic or literary term for someone you are waiting for, often with emotional nuance, whereas 待機者 is neutral and functional.
person self-quarantining at home
Compound of 待機 (standby, waiting) and 者 (person). The quarantine sense is a modern extension of the waiting concept.