An acronym is an abbreviation pronounced as a word—NASA or UNESCO, for instance. The Brits tend to style these sorts of things as Nasa and Unesco and, worst of the worst, Aids, which makes my teeth itch. Once an acronym turns into a common word—likely you’ve forgotten that “radar” is short for “RAdio Detecting And Ranging” (how could you not forget that?) and that “laser” stands for “Light Amplification by Stimulated Emission of Radiation” (ditto)—one drops the capital letters altogether, as I’ve just done. An initialism is an abbreviation pronounced letter by letter—as, say, FBI or CIA.
