An important concept in historical linguistics is cognacy. Words are said to be cognates if they descend directly from the same word in an earlier stage of the same language or in an ancestral language.
For example, the German word Feuer (‘fire’) and English fire are cognates. They both derive from the corresponding word *fōr in the Proto-Germanic language. That word itself originated in the more ancient Proto-Indo-European word reconstructed as *péh₂u̯r̥.
The asterisk shows that the ancestral words have been reconstructed by historical linguists and are not supported by documentary evidence. Indeed, those ancestral languages were not yet recorded in writing.
For discussion of a database of cognates in Indo-European languages, please see https://languagemiscellany.com/2024/03/database-of-indo-european-cognates/
Cognate objects
The word cognate also features in the phrase cognate object. This is an object of a verb having the same origin and a similar meaning. An example of a cognate object is song in sing a song.
Etymon and reflex
Two other words connected with cognate are seen less often but can be useful:
- an etymon of a word (plural: etyma) is its historical ancestor. For example, *fōr is the (reconstructed) Proto-Germanic etymon of German Feuer and English fire. And *péh₂u̯r̥ is the (reconstructed) Proto-Indo-European etymon of Proto-Germanic*fōr, German Feuer and English fire.
- a reflex of a word is a direct descendant of that word in a later known (or reconstructed) language. German Feuer and English fire are reflexes of Proto-Germanic *fōr and of Proto-Indo-European *péh₂u̯r̥.
Cognates are not borrowed
Cognates descend directly from the same etymon, without borrowing.
Many English words descend from Proto-Indo-European, but only indirectly because they were borrowed from, for example, French, Latin or Greek.
An example of a borrowing is Russian фрукты (/frukty/ ‘fruit’), borrowed from Latin frūctus (from PIE *bʰreu̯H⁽g̑⁾-) possibly via Polish, German or Dutch. https://languagemiscellany.com/2026/03/where-did-russian-words-come-from-4/
In historical linguistics, distinguishing cognates from borrowings is important. That is because borrowed words do not undergo processes (such as sound changes) within the recipient language until the borrowing occurs.