Lexicology is the part of linguistics that studies words. This may include their nature and function as symbols, their meaning, the relationship of their meaning to epistemology in general, and the rules of their composition from smaller elements (morphemes such as the English -ed marker for past or un- for negation; and phonemes as basic sound units).Lexicology also involves relations between words, which may involve semantics (for example, love vs. affection), derivation (for example, fathom vs. unfathomably), use and sociolinguistic distinctions (for example, flesh vs. meat), and any other issues involved in analyzing the whole lexicon of a language.
The term first appeared in the 1970s, though there were lexicologists in essence before the term was coined. Computational lexicology is a related field (in the same way that computational linguistics is related to linguistics) that deals with the computational study of dictionaries and their contents.
An allied science to lexicology is lexicography, which also studies words, but primarily in relation with dictionaries – it is concerned with the inclusion of words in dictionaries and from that perspective with the whole lexicon. Sometimes lexicography is considered to be a part or a branch of lexicology, but properly speaking, only lexicologists who actually write dictionaries are lexicographers. Some consider this a distinction of theory vs. practice.
The word "lexicology" derives from the Greek λεξικόν lexicon, neut. of λεξικός lexikos, "of or for words", from λέξις lexis, "speech", "word" (in turn from λέγω lego "to say", "to speak") and -λογία -logia, "the study of", a suffix derived from λόγος logos, amongst others meaning "speech, oration, discourse, quote, study, calculation, reason", in turn also from λέγω.