If you pretend you're sick to stay home from school and then sneak out to a baseball game, that's truancy, meaning it’s an unauthorized absence.
In Middle English, truant — the root of truancy — referred to a vagabond or an idler. That meaning still holds true: if you’re accused of truancy, some may say you’re a vagabond or an idler — meaning you’re shirking your duties. Truancy can refer to any absence without permission, but it often refers to being absent from school. If you stay home from school without good reason, you might be hauled into the principal’s office to explain your truancy.