An acronym is a word or name formed from the initial components of a longer name or phrase. Acronyms are usually formed from the initial letters of words, as in NATO (North Atlantic Treaty Organization), but sometimes use syllables, as in Benelux (short for Belgium, the Netherlands, and Luxembourg). They can also be a mixture, as in radar (RAdio Detection And Ranging).
The broader sense of acronym—the meaning of which includes terms pronounced as letters—is sometimes criticized, but it is the term's original meaning and is in common use. Dictionary and style-guide editors are not in universal agreement on the naming for such abbreviations, and it is a matter of some dispute whether the term acronym can be legitimately applied to abbreviations which are not pronounced "as words", nor do these language authorities agree on the correct use of spacing, casing, and punctuation.
Abbreviations formed from a string of initials and usually pronounced as individual letters are sometimes more specifically called initialisms or alphabetisms; examples are FBI from Federal Bureau of Investigation, and e.g. from Latin exempli gratia.