absentee
A person who is not present where they should be.
An absentee is someone who should be present in a particular place but isn't there.
Most often, you'll encounter this word in school contexts. When your teacher takes attendance and marks you absent, you become an absentee for that day. Schools track absenteeism (the pattern of being absent) because missing too many days makes learning harder.
The word also appears in absentee ballot, which is how people vote when they can't be at their polling place on election day. Maybe they're traveling, living abroad, or simply unable to get there. They fill out their ballot and mail it in ahead of time, ensuring their voice is still heard even though they're absent from the voting location.
You might also hear about an absentee landlord, someone who owns property but lives far away and rarely visits it. The key idea is always the same: the person has a role or responsibility somewhere but physically isn't there. An absentee from band practice misses rehearsal. An absentee from a family gathering doesn't show up to the reunion. The word describes being away from somewhere you're expected or supposed to be.