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 PIE: *nókʷts]], "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 words

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 *gPIE: ʷou-, cow having developed through the Germanic language family while beef has arrived in English from the Italo-Romance family descent. (ModE cow < ME cou < OE cū < PIE *gPIE: ʷou > Latin bov- (stem; dictionary form is bos, borrowed from some Osco-Umbrian dialect) > OFr boef > ME beef)

Cognates across languages

Examples 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) PIE: *nókʷts]], "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), stoorei (Pashto), seren (Welsh), steren (Cornish), estel (Catalan), estrella (Spanish), estrela (Portuguese and Galician) and estêre (Kurdish), from the PIE PIE: *h₂stḗr]], "star".