The term in the title of this article has two meanings. The definition of vocabulary can only be two-sided: it is both a linguistic phenomenon and the science that studies it.
What is vocabulary
Firstly, vocabulary is a certain collection of words of a certain language, a work (at the same time they also speak of a thesaurus), a person. In the latter case, one often encounters the methodological term "vocabulary."
Secondly, lexicology is a science that studies words. You can study them from many different angles: this is both observing how the fund of units of a certain language is changing, and how similar or different similar dialects are, and how each of the tokens functions. There are also some general questions. They relate to words in general.
Science of the word
The word can be studied from many different angles. Morphology considers its grammatical meaning, morphemic and word formation - its morphemic composition and model. Vocabulary is a science that studies a word as such: its meaning, history, relationships with other language units.
Words (or lexical units, tokens) in a language are an interesting, lively and in their own way harmonious system. Some of them are very similar to others in meaning and often interchangeable (this is a synonymous system), others, on the contrary, are opposite in meaning (these are antonyms and close, similar phenomena). Some may coexist in a single text, others are so contradictory to each other that their use in one text looks strange or funny. In some cases, it is appropriate to use some units, in others their use is categorically unacceptable (stylistics deals with these issues). Therefore, vocabulary as a science does not study just every word individually, but the system itself, its rules, laws, norms and potential.
Vocabulary Sections
As you can see, vocabulary is a science with an extensive sphere of interests, therefore, every aspect of looking at a word or at a dictionary system studies its special section. First of all, among them are onomasiology (the science of “naming”, or “nomination”), semantics and semasiology (within their framework, the meaning, meaning, its structure are investigated), phraseology (studying the same language phenomenon), onomastics (considering existing names ), etymology (the science of the origin of words), stylistics (it was already mentioned) and lexicography (focuses on the description of vocabulary, compiling dictionaries).
Some of these sections will be discussed in more detail later in this article.
Phraseology
Vocabulary is a science that studies not only words, but also their combinations, which are strong associations. These "unity" are so indivisible that they function in speech almost on a par with words. Such combinations are called phraseological units.
Everyone is familiar with such expressions as “two boots of steam”, “scrubbing around the barn”, “do not drive a nail into the house”, etc. Each of the words in such combinations has practically lost its own meaning. Without knowing the meaning of phraseologism, it is often impossible to “deduce” it from the terms of the “components” that make up it (since these are not quite lexemes, linguists speak specifically about the “components” of phraseology). Such formations are often perplexed by foreigners precisely because their meaning is not the sum of the meanings of words, but rather, either a story twisted into expression or an artistic image. The history of the appearance of phraseological units is one of the most popular and interesting areas of research.
These strong combinations, existing along with lexemes, are also in close relationships with each other and with lexical units. Thus, to be precise, vocabulary is a science that studies words and phraseological units.
Etymology
Each phraseologism has its own story, but also every lexeme. Vocabulary is a science that studies both the present and the past of words. Etymology examines the origin of linguistic units. For specialists in this field, it is important why it was this combination of sounds that once became known (called) a particular phenomenon. Maybe this word was once borrowed from some language? Which one? Perhaps, over time, the lexeme is so distorted, it began to sound so unlike its original version that modern carriers ceased to see its connection with cognate words, ceased to realize that it was based on the well-known root. The etymological meaning (the meaning that was put into the lexeme at the time it appeared in the language) is often surprising and can shed light on the true meaning of the lexical unit; it often not only makes one think about the word, but also reconsiders its attitude to the phenomenon that it denotes.
Any version of the origin of a word should be based on a masterly knowledge of the history of the language. Unfortunately, in recent years, despite the development of linguistics and the accumulated historical data, cases of arbitrary interpretation of the appearance of lexemes are not uncommon. Often, unprofessional etymologization serves as a tool for ideologists who bring a “scientific basis” to their social and political concepts.
Semantics and Semasiology
In the framework of semantics, vocabulary is the science of the meaning of a word. "Semes" are some minimal elements of meaning. The meaning of each token is their unique combination. It is especially interesting to compare the structure of the meaning of analog words from different languages. The discrepancies in the structure and composition of these minimal elements of meaning not only vividly show how different seemingly similar words of different dialects are, but also clarify how dissimilar in mentality people could “invent” them.
Of course, despite the “specialization”, all these branches of science are closely interconnected and study, in fact, one phenomenon, emphasizing its different sides, and it is the combination of many approaches that makes vocabulary one of the most fascinating and rapidly developing areas of linguistics.