How to call in English words which coincidentally happen to share the same pronunciation and the same meaning but belong to two distinct languages that are linguistically unrelated? I'm thinking of the word 'earth', among a few other words, which is pronounced 'arth' in Arabic and means the same thing as in English.
They are called false cognates. From the Wikipedia:
False cognates are pairs of words that seem to be cognates because of similar sounds and meaning, but actually have different etymologies; these word pairs can be within the same language or be from different ones. This is different from false friends, which may in fact be related but have different meanings. Even though false cognates lack a common root, there may still be an indirect connection between them (for example through phono-semantic matching or folk etymology).
Some examples given in the same page are:
- English ache and Ancient Greek ἄχος (ákhos) (pain, distress)
- English bad, Persian bad, and Armenian vad (ւադ) (bad)
- English chill, chilly and Quechua chiri, chili "cold"
- English cut and French couteau "knife"
- English day, daily and Spanish día (day) (or Latin dies (day) or even English diary
- French feu (fire) and German Feuer (fire)
- German haben and Latin habere (both "have")
- Hindi sant and English saint
A term used in biology but also applicable to linguistics is convergent evolution:
Pinker and Bloom (1990) and others assume that linguistic universals provide evidence for a language faculty, but if languages evolve to adapt to the inductive bias in the human learning procedure, then linguistic universals need not be genetically-encoded constraints, but instead may just be a consequence of convergent evolution towards more learnable grammatical systems. Again to quote Deacon (1997:116) "universal[s]... emerged spontaneously and independently in each evolving language, in response to universal biases in the selection processes affecting language transmission. They are convergent features of language evolution in the same way that the dorsal fins of sharks, ichthyosaurs, and dolphins are independent convergents adaptations of aquatic species."
Briscoe, T. (Ed.). (2002). Linguistic evolution through language acquisition. Cambridge University Press.
Convergent evolution would imply there were some forces that shaped a given word so it would sound similar across different languages. Some examples are the interjection Huh? or the indefinite article an in English and Yiddish (according to this question). "Mama" and "papa" were also thought to be the result of linguistic convergence, although such idea has been criticised (see The Age of Mama and Papa, by Matthey and Bancel). If there's no evidence both words evolved from different origins but got to be similar because of a similar evolutionary pressure, then false cognates is the best term.
You are looking for the very interesting term false friends. If the two words meant the same thing, they would be called cognates. But two words from different languages, with different meanings, but which sound the same, are called false friends.
EDIT: I'll agree that false cognates seems correct. What I know for sure is that I misinterpreted "linguistically unrelated" and read "linguistically related." My fault. Sorry if I misled you.