cognatus

Latin cognatus combined co (together) and gnatus (born), from the root nasci (to be born). A cognatus was a blood relative — someone born from the same stock, related through the mother's side (as opposed to agnatus, who was related through the father's line). The word described genetic relationship: shared origin, common ancestry, the same birth.

Historical linguistics adopted cognate in the 19th century to describe words in different languages that descended from the same proto-language ancestor. English night, German Nacht, Latin nox, Greek nyx, and Sanskrit nakta are cognates: all descended from Proto-Indo-European *nókʷts. The blood-relationship metaphor was precise — these words are related through descent, like members of the same family separated across generations.

The systematic comparison of cognates was the foundation of comparative linguistics, which established the family relationships of the world's languages in the 19th century. William Jones, in a famous 1786 lecture in Calcutta, observed the structural similarities between Sanskrit, Greek, and Latin and proposed their common origin. Franz Bopp, Rasmus Rask, and Jacob Grimm developed the rigorous methods for identifying true cognates versus false ones (false cognates or false friends are words that look similar but have unrelated origins).

Today identifying cognates is a basic strategy for language learners: English speakers learning Spanish, French, or Italian can often recognize cognates of Latin origin. English science — Spanish ciencia, French science, Italian scienza — are cognates, all from Latin scientia. The family resemblance that cognatus described in Roman law describes the family resemblance of words across centuries.