Semantic progression describes the evolution of word usage — usually to the point that the modern meaning is radically different from the original usage.
Examples
demagogue - Originally meant "a popular leader". It is from the Greekdemagogos (leader of the people), from demos (people) + agogos (leader). Now the word has strong connotations of a politician who panders to emotions and prejudice.
democrat - At the time of the American Revolution, the term "democrat" had all the negative connotations of the modern usage of the word "demagogue". A century later, the term had shifted in meaning enough that it was viewed favorably as the name of a national political party.
egregious - Originally described something that was remarkably good. The word is from the Latinegregius (outstanding) which is from e-, ex- (out of) + greg- or grex (flock). Now it means something that is remarkably bad or flagrant.
guy - Guido (Guy) Fawkes was the alleged leader of a plot to blow up the EnglishHouses of Parliament on 5 November1605. The burning on 5 November of a grotesque effigy of Fawkes, known as a "guy," led to the use of the word "guy" as a term for any "person of grotesque appearance" and then to a general reference for a man, as in "some guy called for you." In the 20th century, under the influence of American popular culture, "guy" gradually replaced "fellow," "bloke," "chap" and other such words throughout the English-speaking world, and is also referred to both genders (ie., "Come on you guys!" could refer to a group of men and women).
In general, Semantics (from the Greek semantikos, or "significant meaning," derived from "sema," sign) always refers to some kind of meaning (of something that is written) and is thus usually opposed to syntax, which refers to the formal way in which something is written.
It is a subfield of linguistics that is traditionally defined as the study of meaning.
Semanticprogression describes the evolution of word usage - usually to the point that the modern meaning is radically different from the original usage.
Semantics is often opposed to syntax, in which case the former pertains to what something means while the latter pertains to the formal structure/patterns in which something is expressed (for example written or spoken).
Semantics is distinguished from ontology (study of existence) in being about the use of a word more than the nature of the entity referenced by the word.
Semantics is a subfield of linguistics that is traditionally defined as the study of meaning of (parts of) words, phrases, sentences, and texts.