Semantic drift, in historical linguistics, is a phenomenon whereby words change in meaning over a period of time, resulting in semantic differences between cognates. Historical linguistics (also diachronic linguistics or comparative linguistics) is primarily the study of the ways in which languages change over time, by means of examining languages which are recognizably related through similarities such as vocabulary, word formation, and syntax, as well as the surviving records of ancient languages. ... In general, semantics (from the Greek semantikos, or significant meaning, derived from sema, sign) is the study of meaning, in some sense of that term. ... Cognates are words that have a common origin. ...

For instance, the English word to starve is cognate with the German sterben ("to die"). Though both words arose from a common West Germanic root *sterb-a- ("to die"), and their meanings are still somewhat related, semantic drift has caused their specific meanings to differ. The same may occur language-internally, especially when one form is specifically agglutinated. For example, English to hurdle is cognate to hard and is agglutinated with the -le frequentative suffix. The English language is a West Germanic language that originates in England. ... West Germanic is the largest branch of the Germanic family of languages, including such languages as English, Dutch, and German. ... In grammar, a frequentative form of a word is one which indicates repeated action. ...

A more extreme example is with the English word black, which is cognate with Slavic words for white (Russian белый) the reconstructed Proto-Indo-European root for both is *bhel. English black derives from Germanic *blakaz, a past participle of a verb meaning "to blaze." As an adjective, the word would indicate something that has burned and since what is burnt is generally black, the shift in meaning makes more sense.

Reference

• Jeffers, Robert J. and Lehiste, Ilse (1979). Principles and Methods for Historical Linguistics, MIT press.