Modal words are words in a language that express modality, i.e., possibility, necessity, or contingency.1 One kind of modal word is the modal verb (should, can, might, and ought, as well as oblige, need, and require). Other types of modal words in English include modal adjectives (likely, probable, necessary), modal adverbs (probably, perhaps, certainly), modal prepositions (despite, unless, if), and modal nouns (possibility, probability, certainty).
References
References
- Matthews, Peter (2003). The Concise Oxford Dictionary of Linguistics. Oxford: Oxford University Press.