Cognates in linguistics are words that have a common origin. They may occur within a language, such as shirt and skirt as two English words descended from the Proto-Indo-European word *sker-, meaning "to cut". They may also occur across languages, e.g. night and German Nacht as descendants of Proto-Indo-European *nokt-, "night".
The word cognate derives from Latin cognatus, from co (with) +gnatus, natus, past participle of nasci "to be born". Literally it means "related by blood, having a common ancestor, or related by an analogous nature, character, or function".
The term cognate is not normally used with loanwords. For example, linguists would not say that the English word sushi is cognate to the Japanese word sushi, because the word was borrowed from Japanese into English.
Characteristics of cognate wordsCognates need not have the same meaning: dish (English) and Tisch ("table", German), or starve (English) and sterben ("die", German), or head (English) and chef ("chief, head", French), serve as examples as to how cognate terms may diverge in meaning as languages develop separately, eventually becoming false friends.
In addition to having separate meanings, cognates through processes of linguistic change may no longer resemble each other phonetically: cow and beef both derive from the same Indo-European root *gou-, cow having developed through the Germanic language family while beef has arrived in English from the Italo-Romance family descent. (ModE cow Latin bov- (stem; dictionary form is bos) > OFr boef > ME beef)
Cognates across languagesExamples of cognates in Indo-European languages are the words night (English), nuit (French), Nacht (German), nacht (Dutch), nicht (Scots), natt (Swedish, Norwegian), nat (Danish), noc (Czech, Polish), ночь, noch (Russian), нощ, nosht (Bulgarian), ніч, nich (Ukrainian), ноч, noch/noč (Belarusian) noć (Croatian, Serbian), νύξ, nyx (Greek), nox (Latin), nakt- (Sanskrit), natë (Albanian), noche (Spanish), nos (Welsh), noite (Portuguese and Galician), notte (Italian), nit (Catalan), noapte (Romanian), nótt (Icelandic), and naktis (Lithuanian), all meaning "night" and derived from the Proto-Indo-European (PIE) *nokt-, "night."
Another Indo-European example is star (English), str- (Sanskrit), astre or étoile (French), αστήρ (astēr) (Greek), stella (Latin, Italian), stea (Romanian and Venetian), stairno (Gothic), astl (Armenian), Stern (German), ster (Dutch and Afrikaans), starn (Scots), stjerne (Norwegian and Danish), stjarna (Icelandic), stjärna (Swedish), setare (Persian), seren (Welsh), steren (Cornish), estel (Catalan), estrella (Spanish), estrela (Portuguese and Galician) and estêre (Kurdish), from the PIE *stēr-, "star".
The Hebrew shalom, the Arabic salaam and the Amharic selam ("peace") are also cognates, derived from a common Semitic root, having the triliteral slm.
Cognates may often be less easily recognised than the above examples and authorities sometimes differ in their interpretations of the evidence. The English word milk is clearly a cognate of German Milch and of Russian moloko (<PIE *melg-, "to milk"). On the other hand, French lait and Spanish leche (both meaning "milk") are less obviously cognates of Greek galaktos (genitive form of gala, milk) (<*g(a)lag-, galakt-), as is the English word lactic.
Cognates within the same languageCognates can exist within the same language. For example, English ward and guard (<PIE *wer-, "to perceive, watch out for") are cognate as are shirt and skirt (<PIE *sker-, "to cut"). In some cases, such as "shirt" and "skirt", one of the cognate pairs has an ultimate source in another language related to English, while the other one is native, as happened with many loanwords from Old Norse (which was mutually intelligible with Old English) borrowed when the Vikings conquered part of England. Sometimes, both cognates come from other languages, often the same one but at different times. For example, the word chief comes from the Middle French chef, and its modern pronunciation preserves the Middle French consonant sound. The word chef was borrowed from the same source centuries later, by which time the consonant had changed to a "sh"-sound in French. Such words are said to be etymological twins.
False cognatesFalse cognates are words that are commonly thought to be related (have a common origin) whereas linguistic examination reveals they are unrelated. Thus, for example, on the basis of superficial similarities one might suppose that the Latin verb habere and German haben, both meaning 'to have', are cognates. However, an understanding of the way words in the two languages evolve from Proto-Indo-European (PIE) roots shows that they cannot be cognate (see for example Grimm's law). German haben (like English have) in fact comes from PIE *kap, 'to grasp', and its real cognate in Latin is capere, 'to seize, grasp, capture'. Latin habere, on the other hand, is from PIE *ghabh, 'to give, to receive', and hence cognate with English give and German geben.
The similarity of words between languages is not enough to demonstrate that the words are related to each other, in much the same way that facial resemblance does not imply a close genetic relationship between people. Over the course of hundreds and thousands of years, words may change their sound completely. Thus, for example, English five and Sanskrit pança are cognates, while English over and Hebrew a'var are not, and neither are English dog and Mbabaram dog.
Contrast this with false friends, which frequently are cognate.
Parliamentary termIn a parliamentary sense, a cognate debate means that two or more bills can be debated together, if the House does not object to the matter. Bills are only debated cognately if they are closely related.
Mechanical systemsIn Mechanical Systems, the term "cognate" has been used by Hartenburg and Denavit to describe a linkage, of different geometry, which generates the same coupler curve.
Molecular Biology termIn molecular biology a ligand may have a cognate receptor. This is a receptor that specifically binds to that ligand.
cognation in Afrikaans: Kognaat
cognation in Bulgarian: Когнат
cognation in Spanish: Cognado
cognation in French: Mots apparentés
cognation in Galician: Cognado
cognation in Korean: 동계어
cognation in Indonesian: Kata kerabat
cognation in Dutch: Cognaat (taalkunde)
cognation in Japanese: 同根語
cognation in Norwegian: Kognat (lingvistikk)
cognation in Norwegian Nynorsk: Kognat i lingvistikken
cognation in Portuguese: Cognato
cognation in Scots: Cognate
cognation in Finnish: Sukulaissana
cognation in Swedish: Kognat (lingvistik)