Semasiology (from Greek: σημασία, semasia, "signification") is a discipline of linguistics concerned with the question "what does the word X mean?". It studies the meaning of words regardless how they are pronounced. 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.
The exact meaning of semasiology is somewhat obscure. It is often used as a synonym of semantics