Not to be mistaken with lexicography.
Lexicology (from lexiko-, in the Late Greek lexikon) is that part of linguistics, which studies words, their nature and meaning, words' elements, relations between words (semantical relations), words groups and the whole lexicon.
The term first appeared in the 1820s, though there were lexicologists in the straight meaning even before that. Computational lexicology as a related field (in the same way that computational linguistics is related to linguistics) deals with the computational study of dictionaries and their contents. An allied science to lexicology is lexicography, which also studies words in relation with dictionaries - it is actually concerned with the inclusion of words in dictionaries and from that perspective with the whole lexicon. Therefore lexicography is the theory and practice of composing dictionaries. Sometimes lexicography is considered to be a part or a branch of lexicology, but the two disciplines should not be mistaken: lexicographers are the people who write dictionaries, they are at the same time lexicologists too, but not all lexicologists are lexicographers! It is said that lexicography is the practical lexicology, it is practically oriented though it has its own theory, while the pure lexicology is mainly theoretical.

Semantics. Lexical semantics
Semantical relations between words are manifested in respect of homonymy, antonymy, paronymy, etc. Semantics usually involved in lexicological work is called lexical semantics. Lexical semantics is somewhat different from other linguistic types of semantics like phrase semantics, semantics of sentence, and text semantics, as they take the notion of meaning in much broader sense. There are outside (although sometimes related to) linguistics types of semantics like cultural semantics and computational semantics, as the latest is not related to computational lexicology but to mathematical logic. Among semantics of language, lexical semantics is most robust, and to some extend the phrase semantics too, while other types of linguistic semantics are new and not quite examined.

The domain of Lexical Semantics
Lexical semantics may not be understood without a brief exploration of its history.
Prestructuralist semantics
Semantics as a linguistic discipline has its beginning in the middle of the 19th century, and because linguistics at the time was predominantly diachronic, thus lexical semantics was diachronic too - it dominated the scene between the years of 1870 and 1930., these were interpretative and generative semantics.
Cognitive semantics

Main article: Cognitive semantics History of Lexical Semantics

Main article: Phraseology Phraseology

Main article: Etymology Etymology

Main article: Lexicography Lexicology in life: Lexicography

Main article: Lexicographer