This lecture is devoted to lexical semantics, which is the branch of linguistics which is concerned with the systematic study of word meanings. Probably the two most fundamental questions addressed by lexical semanticists are: (a) how to describe the meanings of words, and (b) how to account for the variability of meaning from context to context. These two are necessarily connected, since an adequate description of meaning must be able to support our account of variation and our ability to interpret it. Another important area of inquiry is how the vocabularies of languages are structured by means of sense relations (systematic relations between meanings), such as antonymy (long:short, fast:slow), hyponymy (animal:dog, fruit:apple), and incompatibility (dog:cat, apple:banana).