Lexicology is the branch of linguistics that studies the stock of words (the lexicon) in a given language. Adjective: lexicological.
From the Greek lexico- + -logy, "word + study"
Lexicology and Syntax
"Lexicology deals not only with simple words in all their aspects but also with complex and compound words, the meaningful units of language. Since these units must be analyzed in respect of both their form and their meaning, lexicology relies on information derived from morphology, the study of the forms of words and their components, and semantics, the study of their meanings. A third field of particular interest in lexicological studies is etymology, the study of the origins of words. However, lexicology must not be confused with lexicography, the writing or compilation of dictionaries, which is a special technique rather than a level of language studies…
"The essential difference between syntax and lexicology is that the former deals with the general facts of language and the latter with special aspects… Syntax is general because it deals with rules and regularities that apply to classes of words as a whole, whereas lexicology is particular because it is concerned with the way individual words operate and affect other words in the same context. Although borderline cases do exist in both lexicology and syntax, e.g., in the case of 'grammatical' or 'function' words, the distinction between the two levels is fairly clear." (Howard Jackson and Etienne Zé Amvela, Words, Meaning, and Vocabulary: An Introduction to Modern English Lexicology. Continuum, 2007)
Content Words and Function Words
"Teachers of English have customarily distinguished between content words, like snow and mountain, and function words, like it and on and of and the… Lexicology is the study of content words or lexical items." (M.A.K. Halliday et al., Lexicology and Corpus Linguistics. Continuum, 2004)
Lexicology and Grammar
"Both grammar and lexicology involve us in an indefinitely large number of superficially different units. In the case of grammar these are phrases, clauses, and sentences; in the case of lexicology the units are words, or more precisely… lexical items. It is typical of grammar to make general and abstract statements about the units concerned, showing a common construction despite formal differences. It is typical of lexicology to make specific statements about individual units. In consequence, while the grammar of a language is best handled in chapters devoted to different types of construction, it is normal to deal with the lexicon of a language in an alphabetical dictionary, each entry devoted to a different lexical item." (Randolph Quirk et al., A Comprehensive Grammar of the English Language, 2nd ed. Longman, 1985)
Lexicology and Phonology
"It may be thought at first sight that phonology does not interact with lexicology in any significant manner. But a close analysis will reveal that, in many cases, the difference between two otherwise identical lexical items can be reduced to a difference at the level of phonology. Compare for example the pair of words toy and boy, feet and fit, pill and pin. They differ only in one sound unit (the position of which has been italicized in each word) and yet the difference has serious consequences at the level of lexicology." (Etienne Zé Amvela, "Lexicography and Lexicology." Routledge Encyclopedia of Language Teaching and Learning, ed. by Michaël Byram. Routledge, 2000)