False cognates are pairs of words in the same or different languages that are similar in form and meaning but have different roots. That is, they appear to be or are sometimes considered cognates when in fact they are not. Note that even false cognates may have an indirect connection between them, even if they lack a common root. Ex: English great and English grand; English aye (yes, affirmative vote) and Japanese hai (yes) and Cantonese "hai" (yes); English bullshit and Mandarin búshì (不是 ; is not, not true); English dork and Russian durak; English can and Japanese kan (cylindrical metal container); English day, daily and Spanish día (day) (Latin dies or English diary); English hut and Russian hata (хата); English male and English female, which come from the Latin masculinus and femella, respectively; English much and Spanish mucho; French papillon (butterfly) and Nahuatl papalotl (butterfly); Greek theos (god) and Latin deus (god).

The term "false cognate" is sometimes misused to describe false friends. False friends (or faux amis) are pairs of words in two languages or dialects (or letters in two alphabets) that look and/or sound similar, but differ in meaning. One difference between false cognates and false friends is that while false cognates mean roughly the same thing in two languages, false friends bear two distinct (sometimes even opposite) meanings. In fact, a pair of false friends may be true cognates (see false friends: causes). English and Romanian contain a fair few false friends, since Romanian is Latin based, and English has tons of Latin based stuff in it.

Exercise: Name the English ‘false friend’ of the following Romanian terms (and wherein the difference of meaning lies).Librărie, nervos, mizerabil, crimă, prezervativ, actual, concurenţă, eventual, hazard, rumoare, fabrică, argument, corp, cameră, mare, lectură, confecţie, gimnaziu, complement, nuvelă, local, probă, rată, reţetă, a absolvi, magazin, fizician, ban, jar, loc, ham, abstract, sensibil, simpatic.

One kind of false friend can occur when two speakers speak different varieties of the same language. Speakers of British English and American English sometimes have this problem, which was alluded to in George Bernard Shaw's statement "England and America are two countries divided by a common language". For example, in the UK, to "table" a motion means to place it on the agenda, while in the U.S. it means exactly the opposite —"to remove it from consideration".

However, false friends have caused accidents and other serious incidents. One of the best-known examples is the 1990 crash of Avianca Flight 52 in New York. As the aircraft was running out of fuel due to its having been on hold too long for landing, the crew asked for "priority" landing instead of "emergency" landing. Unlike English where "priority" is merely part of a sequence of ever more serious grades of urgency rather than an extreme emergency, the Spanish equivalent prioridad indicates that immediate action is required. With no one on the ground realizing the severity of the situation, the plane ran out of fuel and crashed.

Examples of false friends between British and American English: athlete, awesome, bangs/fringes, bathroom, bill, block, bomb, bonnet/hood, boot/trunk, brackets/parentheses, caravan/trailer, carnival, casualty, chap(s), chips, closet, college, corn, cot, court shoe, deadbeat, dummy/pacifier, elevator/lift, fag, fall, fancy dress, first floor, football, full stop/period, gas/petrol, hire/rent, hob/cooktop, interval/intermission, jam/jelly, lavatory, Lecturer, lorry/truck, mad, mate, mean, mobile phone/cell phone, to nick, patience/solitaire, pavement/sidewalk, pissed/smashed, Professor, public school, purse, queue/line, quid/buck, quite, raisin/sultanas, restroom, roommate/housemate, rubber, saloon, set square/triangle, starter/appetizer, store, student, tap/faucet, theatre/operating room, through, tick/check mark, tights/leggings/pantyhose, torch/flashlight, tosser, trainer/sneaker, transport/transportation, trolley, tube/underground/subway, waistcoat/vest, zip

From the etymological point of view, false friends can be created in several ways: a) borrowing (If Language A borrowed a word from Language B, and in one language the word shifted in meaning or had more meanings added); b) homonyms. In certain cases, false friends evolved separately in the different languages; c) pseudo-anglicisms (new words formed from English morphemes independently from an analogous English construct and with a different intended meaning). Ex., in German Oldtimer refers to an old car (or antique aircraft) rather than an old person, while Handy refers to a mobile phone.

False friends resulting in a semantic change in the standard language In bilingual situations, false friends often result in a real new meaning that is commonly used in a language. For example, Portuguese humoroso "capricious" changed its referent in American Portuguese to "humorous" owing to the English surface-cognate "humorous".