English almost never uses this word by itself. You meet it in the Latin phrase in absentia, which means “while the person is not there.” Courts use it: someone can be tried in absentia, or found guilty in absentia, when they should be in the courtroom but are not — they have fled, they are in another country, they refuse to come. An official body can also pass a decision in absentia. Though actually, this is a lawyer’s and news-writer’s phrase, not a word for daily conversation. You would not say “my absentia” for “I wasn’t there.” For ordinary life, say “I was away” or “he missed the meeting.” Keep absentia for that formal setting where a decision is made about you and you are not in the room.