Cognate

In linguistics, cognates, also called lexical cognates, are words that have a common etymological origin. Cognates are often inherited from a shared parent language, but they may also involve borrowings from some other language. For example, the English words "dish", "disk" and "desk" and the German word "Tisch" ("table") are cognates because they all come from Latin "discus", which relates to their flat surfaces. Cognates may have evolved similar, different or even opposite meanings, and although there are usually some similar sounds or letters in the words, they may appear to be dissimilar. Some words sound similar, but do not come from the same root; these are called false cognates, while some are truly cognate but differ in meaning; these are called false friends.
The word "cognate" derives from the Latin noun "cognatus", which means "blood relative".
Characteristics.
Cognates do not need to have the same meaning, which may have changed as the languages developed separately. For example English "starve" and Dutch "sterven" or German "sterben" ("to die") all derive from the same Proto-Germanic root, "*sterbaną" ("die"). "Discus" is from Greek (from the verb "to throw"). A later and separate English reflex of "discus", probably through medieval Latin , is "desk" (see OED s.v. "desk").
Also, cognates do not need to have similar forms: 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" (note that the sound change "*dw" > "erk" in Armenian is regular).
Across languages.
Examples of cognates in Indo-European languages are the words "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), "nishi" (Bengali), "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), "nos" (Welsh, Cornish), "noz" (Breton), "nox/nocte" (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), all meaning "night" and being derived from the Proto-Indo-European "night".
Another Indo-European example is "star" (English), "starn" (Scots), "Stern" (German), "ster" (Dutch and Afrikaans), "stjer" (Frisian) "Schtähn" (Colognian), "stjärna" (Swedish), "stjerne" (Norwegian and Danish), "stjarna" (Icelandic), "stjørna" (Faroese), "stairno" (Gothic), "str-" (Sanskrit), "tara" (Hindustani and Bengali), "tera" (Sylheti), "tora" (Assamese), "setāre" (Persian), "stoorei" (Pashto), "estêre" or "stêrk" (Kurdish), "astgh" (Armenian), "ἀστήρ (astēr)" (Greek or "ἀστέρι"/"ἄστρο", "asteri"/"astro" in Modern Greek), "astrum" / "stellă" (Latin), "astre" / "étoile" (French), "astro" / "stella" (Italian), "stea" (Romanian and Venetian), "estel" (Catalan), "astru" / "isteddu" (Sardinian), "estela" (Occitan), "estrella" and "astro" (Spanish), "estrella" (Asturian and Leonese), "estrela" and "astro" (Portuguese and Galician), "seren" (Welsh), "steren" (Cornish) and "sterenn" (Breton), from the Proto-Indo-European "star".
The Arabic "salām", the Hebrew "shalom", the Assyrian Neo-Aramaic "shlama" and the Amharic "selam" ("peace") are also cognates, derived from the Proto-Semitic *šalām- "peace".
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 and Afrikaans "melk", Russian молоко (moloko), Serbo-Croatian and Slovenian "mleko"/"mlijeko". On the other hand, French "lait", Catalan "llet", Italian "latte", Romanian "lapte", Spanish "leche" and "leite" (Portuguese and Galician) (all meaning "milk") are less-obvious cognates of Ancient Greek "" "gálaktos" (genitive singular of "gála", "milk"), a relationship that is more evidently seen through the intermediate Latin "lac" "milk" as well as the English word "lactic" and other terms borrowed from Latin.
Some cognates are semantic opposites. For instance, while the Hebrew word "chutzpah" means "impudence", its Classical Arabic cognate "ḥaṣāfah" means "sound judgment." Another example is English "empathy" "understanding of thoughts" and Greek "empátheia" "malice".
Within the same language.
Cognates within a single language, or "doublets", may have meanings that are slightly or even 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, including this one, one cognate ("skirt") has an ultimate source in another language related to English, but the other one ("shirt") is native. That happened with many loanwords, such as "skirt" in this example, which was borrowed from Old Norse during the Danelaw.
Sometimes both doublets 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, but by then, the consonant had changed to a "sh" sound in French. Such word sets can also be called etymological twins, and they may come in groups of higher numbers, as with, for example, the words "wain" (native), "waggon/wagon" (Dutch), and "vehicle" (Latin) in English.
A word may also enter another language, develop a new form or meaning there, and be re-borrowed into the original language; that is called reborrowing. For example, the Greek word ("kínima", "movement") became French "cinéma" (compare American English "movie") and then later returned to Greece as ("sinemá", "the art of film", "movie theater"). In Greek, ("kínima", "movement") and ("sinemá", "filmmaking, cinema") are now doublets.
A less obvious English-language doublet pair is "grammar" and "glamour".
False cognates.
False cognates are words that people commonly believe are related (have a common origin), but that linguistic examination reveals are unrelated. For example, on the basis of superficial similarities, the Latin verb "habēre" and German "haben", both meaning 'to have', appear to be cognates. However, because the words evolved from different roots, in this case, different Proto-Indo-European (PIE) roots, they cannot be cognate (see for example Grimm's law). German "haben", like English "have", comes from PIE "*kh₂pyé-" 'to grasp', and its real cognate in Latin is "capere", 'to seize, grasp, capture'. Latin "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, as they evolved from different roots: "much" from Proto-Germanic "*mikilaz" < PIE "*meǵ-" and "mucho" from Latin "multum" < PIE "*mel-". Instead, its real cognate is Spanish "maño".