Absence is the state of not being present somewhere you're expected or supposed to be. When a student is marked absent from school, they're not in their seat when the teacher takes attendance. When someone notices your absence from soccer practice, they realize you're missing.

The word also describes the complete lack of something. Scientists studying a distant planet might note the absence of water or oxygen. A doctor checking a patient's reflexes watches for the presence or absence of certain responses. When we say “in the absence of evidence,” we mean when there's no evidence at all.

Absence carries a sense of something missing that should be there or that we expect to find. It's different from something that was never there in the first place. You wouldn't talk about the absence of dinosaurs at your school because nobody expects dinosaurs there. But you would notice the absence of your best friend, especially if they're usually sitting right next to you.

The phrase “absence makes the heart grow fonder” suggests that being away from someone can make you appreciate them more, though sometimes the opposite proves true: “out of sight, out of mind.”