Lexical semantics is a subfield of linguistic semantics. It is the study of how and what the words of a language denote (Pustejovsky, 1995). Words may either be taken to denote things in the world or concepts, depending on the particular approach to lexical semantics.
The units of meaning in lexical semantics are lexical units, which a speaker can continually add to throughout their life, learning new words and their meanings. By contrast, one can only easily learn the grammatical rules of one's native language during a critical period when one is young.