In historical linguistics, cognates or lexical cognates are sets of words that have been inherited in direct descent from an etymological ancestor in a common parent language. Because language change can have radical effects on both the sound and the meaning of a word, cognates may not be obvious, and often it takes rigorous study of historical sources and the application of the comparative method to establish whether lexemes are cognate. Cognates are distinguished from loanwords, where a word has been borrowed from another language.
The English term cognate derives from Latin cognatus, meaning "blood relative".
Cognates need not have the same meaning, which may have changed as the languages developed independently. For example English starve and Dutch sterven 'to die' or German sterben 'to die' all descend from the same Proto-Germanic verb, *sterbaną 'to die'.
Cognates also do not need to look or sound similar: English father, French père, and Armenian հայր (hayr) all descend directly from Proto-Indo-European *ph₂tḗr. An extreme case is Armenian երկու (erku) and English two, which descend from Proto-Indo-European *dwóh₁; the sound change *dw > erk in Armenian is regular.
An example of cognates from the same Indo-European root are: night (English), nicht (Scots), Nacht (German), nacht (Dutch, Frisian), nag (Afrikaans), Naach (Colognian), natt (Swedish, Norwegian), nat (Danish), nátt (Faroese), nótt (Icelandic), noc (Czech, Slovak, Polish), ночь, noch (Russian), ноќ, noć (Macedonian), нощ, nosht (Bulgarian), nos (Welsh/Cymraeg),ніч, nich (Ukrainian), ноч, noch/noč (Belarusian), noč (Slovene), noć (Serbo-Croatian), nakts (Latvian), naktis (Lithuanian), νύξ, nyx (Ancient Greek), νύχτα / nychta (Modern Greek), nakt- (Sanskrit), natë (Albanian), nox, gen. sg. noctis (Latin), nuit (French), noche (Spanish), nueche (Asturian), noite (Portuguese and Galician), notte (Italian), nit (Catalan), nuet/nit/nueit (Aragonese), nuèch / nuèit (Occitan) and noapte (Romanian). These all mean 'night' and derive from the Proto-Indo-European *nókʷts 'night'. The Indo-European languages have hundreds of such cognate sets, though few of them are as neat as this.
The Arabic سلام salām, the Hebrew שלום shalom, the Assyrian Neo-Aramaic shlama and the Amharic selam 'peace' are cognates, derived from the Proto-Semitic *šalām- 'peace'.
Main article: False cognate
False cognates are pairs of words that appear to have a common origin, but which in fact do not. For example, Latin habēre and German haben both mean 'to have' and are phonetically similar. However, the words evolved from different Proto-Indo-European (PIE) roots: haben, like English have, comes from PIE *kh₂pyé- 'to grasp', and has the Latin cognate capere 'to seize, grasp, capture'. Habēre, on the other hand, is from PIE *gʰabʰ 'to give, to receive', and hence cognate with English give and German geben.
Likewise, English much and Spanish mucho look similar and have a similar meaning, but are not cognates: much is from Proto-Germanic *mikilaz < PIE *meǵ- and mucho is from Latin multum < PIE *mel-. A true cognate of much is the archaic Spanish maño 'big'.
Cognates are distinguished from other kinds of relationships.
An etymon, or ancestor word, is the ultimate source word from which one or more cognates derive. In other words, it is the source of related words in different languages. For example, the etymon of both Welsh ceffyl and Irish capall is the Proto-Celtic *kaballos (all meaning horse).
Descendants are words inherited across a language barrier, coming from a particular etymon in an ancestor language. For example, Russian мо́ре and Polish morze are both descendants of Proto-Slavic *moře (meaning sea).
A root is the source of related words within a language. Similar to the distinction between etymon and root, a nuanced distinction can sometimes be made between a descendant and a derivative. A derivative is one of the words which have their source in a root word, and were at some time created from the root word using morphological constructs such as suffixes, prefixes, and slight changes to the vowels or to the consonants of the root word. For example unhappy, happily, and unhappily are all derivatives of the root word happy.
The terms root and derivative are used in the analysis of morphological derivation within a language, in studies that are not concerned with historical linguistics, and that do not cross the language barrier.
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