Semasiology (from Greek: σημασία (semasia) "signification, meaning") is a discipline within linguistics concerned with the question "what does the word X mean?". It studies the meaning of words regardless of their phonetic expression. Semasiology departs from a word or lexical expression and asks for its meaning, its different senses, i.e. polysemy. The opposite approach is known as onomasiology.
The term was first used by Christian Karl Reisig in 1825 in his Vorlesungen über lateinische Sprachwissenschaft (E. Lectures on Latin Linguistics) and was in use in English by 1847. Semantics replaced it in its original meaning, beginning in 1893.
Currently, the discipline is most commonly understood as a branch of lexicology, the study of words, and as a branch of semantics, and more narrowly ascribed as a subfield of lexical semantics, though sometimes referred to as a synonym of semantics. The exact meaning of the term is somewhat obscure, because according to some authors semasiology merged with semantics in modern times, while at the same time the term is still in use when defining onomasiology.