Staying away from school without permission — skipping school, on purpose, with nobody's agreement. Being sick is not truancy. Being kept home by a parent to help at the family shop is a different offence. Truancy is the child choosing not to go and nobody knowing where they are. The word is official and institutional: it lives in school reports, government statistics, newspaper articles and law. A teacher writes 'truancy' in a file; the same teacher says 'skipping class' to the student's face, and the students themselves say 'cutting class' (American) or 'bagging off / skiving' (British). Because it is the formal word, it usually appears in serious contexts — 'truancy rates', 'a truancy officer', 'measures to reduce truancy' — and often with a whiff of the law behind it, since in many countries parents can be fined for it. The person who does it is a truant, and the older verb phrase is 'to play truant' (British) or 'to play hooky' (American). It is an uncountable noun, so you say 'truancy is a problem', never 'a truancy' or 'truancies'.