What is the study of development words called?

There isn't a single, specific field of study that focuses solely on the development of words. However, depending on what aspects of word development you're interested in, there are several disciplines that overlap:

* Etymology: This field focuses on the origin and historical development of words. It investigates how words change in form and meaning over time.

* Lexicology: This area studies the vocabulary of a language, including the structure, meaning, and history of words.

* Linguistics: This broader field encompasses many areas, including phonetics, morphology, syntax, and semantics. It investigates how language works, including the development of words as part of the overall language system.

* Child Language Acquisition: This field specifically examines how children learn language, including how they acquire vocabulary.

* Cognitive Psychology: This field studies mental processes, including language comprehension and production. It explores how the brain processes words and how our understanding of words develops.

So, depending on the specific angle you're interested in, one or more of these fields could be considered the study of development words.