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 Greek demagogos (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 Latin egregius (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 English Houses of Parliament on 5 November 1605. 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).