radar

RADAR was coined in November 1940 by U.S. Navy officers Samuel M. Tucker and F. R. Furth as an acronym for 'radio detection and ranging' — combining 'radio' (ultimately from Latin radius), 'detection' (from Latin dētēctiōnem), 'and' (Old English and), and 'ranging' (from English range, via Old French rengier).

What began as a functional military acronym, written in capitals to spell out its four components, gradually settled into ordinary lowercase use as 'radar,' losing any sense of being an abbreviation at all.

It shares a root with 'telegraph' and 'radio,' both of which trace back through 'radiotelegraphy' to the Latin radius.