In linguistics, cognates are words that have a common etymological origin. This learned term derives from the Latin cognatus (blood relative). Cognates within the same language are called doublets. Strictly speaking, loanwords from another language are not considered cognates.
For example, the English words shirt and skirt are doublets; the former derives from the Old English sċyrte, while the latter is loaned from Old Norse skyrta, both of which derive from the Proto-Germanic *skurtijǭ. Additional cognates of the same word in other Germanic languages include the German Schürze and Dutch schort (which both mean "apron").
Characteristics of cognate words 
Cognates do not need to have the same meaning, which may have changed as the languages developed separately. For example, consider English starve and Dutch sterven or German sterben ("to die"); these three words all derive from the same Proto-Germanic root, *sterbaną ("die"). English dish and German Tisch ("table"), with their flat surfaces, both come from Latin discus, but it would be a mistake to identify their later meanings.
Cognates across languages 
Examples of cognates in Indo-European languages are the words night (English), nuit (French), Nacht (German), nacht (Dutch), nag (Afrikaans), nicht (Scots), natt (Swedish, Norwegian), nat (Danish), nátt (Faroese), nótt (Icelandic), noc (Czech, Slovak, Polish), ночь, noch (Russian), ноќ, noć (Macedonian), нощ, nosht (Bulgarian), ніч, nich (Ukrainian), ноч, noch/noč (Belarusian), noč (Slovene), noć (Serbo-Croatian), νύξ, nyx (Ancient Greek, νύχτα/nychta in Modern Greek), nox (Latin), nakt- (Sanskrit), natë (Albanian), noche (Spanish), nos (Welsh), nueche (Asturian), noite (Portuguese and Galician), notte (Italian), nit (Catalan), nuèch/nuèit (Occitan), noapte (Romanian), nakts (Latvian) and naktis (Lithuanian), all meaning "night" and derived from the Proto-Indo-European (PIE) *nókʷts, "night".
Another Indo-European example is star (English), str- (Sanskrit), tara (Hindustani), étoile (French), ἀστήρ (astēr) (Greek or ἀστέρι/ἄστρο, asteri/astro in Modern Greek), stella (Italian), aster (Latin) stea (Romanian and Venetian), stairno (Gothic), astl (Armenian), Stern (German), ster (Dutch and Afrikaans), starn (Scots), stjerne (Norwegian and Danish), stjarna (Icelandic), stjärna (Swedish), stjørna (Faroese), setāre (Persian), stoorei (Pashto), seren (Welsh), steren (Cornish), estel (Catalan), estela (Occitan) estrella Spanish, estrella Asturian and Leonese, estrela (Portuguese and Galician) and estêre or stêrk (Kurdish), from the PIE *h₂stḗr, "star".
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, Dutch melk, Russian молоко (moloko) and Croatian mlijeko. On the other hand, French lait and Spanish leche (both meaning "milk") are less obviously cognates of Ancient Greek γάλακτος gálaktos (genitive singular of γάλα gála, "milk"), a relationship more evidently seen through the intermediate Latin lac "milk", as well as the English word lactic and other terms borrowed from Latin. At times, cognates may even be opposites. For instance, while the Hebrew word חוצפה chutzpah means "impudence," its Classical Arabic cognate حصافة ḥaṣāfah means "sound judgment;" even more contradictorily, the English word black and Polish biały, meaning white, both derive from the PIE *bʰleg-, meaning, "to burn or shine."
A word may also enter another language, develop a new form or meaning there, and be re-borrowed into the original language; this is called a Rückwanderer (German for "one who wanders back"). For example, the Greek word κίνημα (kinēma, "movement") became French cinéma (cf. American English movie) and then later returned to Greece as σινεμά (sinema, "the art of film", "movie theater"). Now in Greece κίνημα (kinēma, "movement") and σινεμά (sinema, "filmmaking, cinema") exist together as a doublet (see next section).
Cognates within the same language 
Cognate doublets can exist within the same language, with meanings which may be anything from slightly to totally different. For example, English ward and guard (<PIE *wer-, "to perceive, watch out for") are cognates, as are shirt (garment on top) and skirt (garment on bottom) (<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 borrowed during the Danelaw. Sometimes, both cognates come from other languages, often the same one but at different times. For example, the word chief (meaning the leader of any group) comes from the Middle French chef ("head"), and its modern pronunciation preserves the Middle French consonant sound; the word chef (the leader of the cooks) was borrowed from the same source centuries later, by which time the consonant had changed to a "sh"-sound in French. Such word sets can also be called etymological twins, and of course they may come in groups of higher numbers, as with, for example, the words wain (native) wagon (Dutch) and vehicle (Latin) in English.
An example of very different and non-obvious English-language cognates is grammar and glamour.
False cognates 
False 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', were 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 *kh₂pyé- '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. English much and Spanish mucho also look similar and even have a similar meaning yet are not cognates, with much < Proto-Germanic *mikilaz < PIE *meg-, while mucho < Latin multum < PIE *mel-.
Similarity of words between languages does not prove that the words are related, in much the same way as facial resemblance does not prove relatedness of people. Over the course of hundreds and thousands of years, words may change their sound completely. For example, English over and Hebrew ′avar are not cognates, and neither are English dog and Mbabaram dog. False cognates arise in the same way as false or folk etymology, spurious explanations for the origin of words.
Contrast this with false friends, which frequently are cognate.
See also 
|Look up cognate in Wiktionary, the free dictionary.|
- "cognate", The American Heritage Dictionary of the English Language, 4th ed.: "Latin cognātus: co-, co- + gnātus, born, past participle of nāscī, to be born." Other definitions of the English word include "[r]elated by blood; having a common ancestor" and "[r]elated or analogous in nature, character, or function". Ibid.
- Discus is itself from Greek δίσκος (from the verb δικεῖν "to throw"). A later and separate English reflex of discus, probably through medieval Latin desca, is desk (see OED s.v. desk).
- Cf. also Greek ἀμέλγω amelgō "to milk".
- Wehr, Hans (1994) . In J. Milton Cowan. Dictionary of Modern Written Arabic. Urbana, Illinois: Spoken Language Services, Inc. ISBN 0-87950-003-4.
- In fact, σινεμά stands beside a Greek neologism based on the original form of the same root: κινηματογράφος (kinimatoγráfos), with the same two meanings as cinéma/σινεμά. (The film or movie itself is the unrelated ταινία (tainia).
- Ringe, Don. A quick introduction to language change, 22 March 2010