This is a fine distinction that some consider nitpicking. Terms such as FBI, HMO, and NAACP, although widely called acronyms, are actually abbreviations. The difference is in how they are spoken. An abbreviation, also called an initialism, is pronounced letter by letter. An acronym is pronounced as if it were a word. The abbreviation FBI is pronounced "eff-bee-eye." The acronym NASA is pronounced "nassa."