A modality is the way or mode in which something exists or is done. You might often see it used with reference to diagnostic modality, which is the way in which a disease or illness is diagnosed by a doctor.
Modality shares its root with the word mode, meaning "the way in which something happens or is experienced." A sensory modality is a way of sensing, like vision or hearing. Modality in someone's voice gives a sense of the person's mood. In logic, modality has to do with whether a proposition is necessary, possible, or impossible. In general, a modality is a particular way in which something exists.