Linguistics is the scientific study of language. Linguistics examines language through its structue and use. the fields of structure are Phonetics & Phonology, Morphology, Syntax, Semantics, and Pragmatics which deal with sounds, words, sentences, meaning, and interaction respectively. Language use is studied as it interface with other fields such as sociology, psychology, computer science, literature, anthropology, philosophy, or history.
Isn't John studying linguistics?
No, he just speaks four langauges, but Jill is a linguist.