Semasiology (from Greek: σημασία, semasia, "signification") is a discipline of linguistics concerned with the question "what does the word X mean?".
[1] It is the opposite of onomasiology, a branch of lexicology that starts with a concept or object and asks for its name, i.e., "how do you express X?"
whereas semasiology starts with a word and asks for its meanings.
It is often used as a synonym of semantics (the study of the meaning of words, phrases, and longer forms of expression).
The term was first used in German by Christian Karl Reisig in 1825 in his work, [Lectures on Latin Linguistics] (German: Vorlesungen über lateinische Sprachwissenschaft), and was used in English by 1847.