Linguistic modality, the subject concerning so-called modal auxiliary verbs like can, must, and should, customarily used to modify the meaning of other verbs, to express possibility (and impossibility, necessity, contingency, etc.), permissibility (and obligation, proscription, etc.), probability (and improbability, etc.). Compare grammatical mood.
Stimulus modality, a type of physical phenomenon that one can sense, such as temperature and sound.