Etymology is the study of the origin and evolution of words—including their constituent units of sound and meaning—across time. In the 21st century, as a subfield within linguistics, etymology has become an increasingly rigorous scientific field of study. It is most directly tied to historical linguistics, philology, and semiotics, and additionally draws upon comparative semantics, morphology, pragmatics, and phonetics in order to construct a comprehensive and chronological catalogue of all meanings that a word has carried throughout its history. The origin of any particular word is also known as its etymology.