Abbreviations, acronyms and initialisms are all types of shortened forms of words or phrases. An abbreviation is a shortened form of a word or phrase, such as “Dr.” for “Doctor”. An acronym is an abbreviation formed by combining the first letters of a string of words, and is pronounced as a word, such as LASER (Light Amplification by Stimulated Emission of Radiation). An initialism is an abbreviation formed by combining the first letters of a string of words, but it is pronounced one letter at a time, such as FBI (Federal Bureau of Investigation).An example of an initialism with more than three letters is NAACP (National Association for the Advancement of Colored People).
The difference between abbreviations, acronyms and initialisms lies in how they are pronounced. Acronyms are pronounced as words, while initialisms are pronounced one letter at a time. Abbreviations can be either. For example, NASA (National Aeronautics and Space Administration) is an acronym because it is pronounced as a word, while FBI and CIA (Federal Bureau of Investigation and Central Intelligence Agency) are initialisms because they are pronounced one letter at a time. Other examples of initialisms include FYI (For Your Information) and PR (Public Relations).In summary, an abbreviation is a truncated word; an acronym is made up of parts of the phrase it represents and is pronounced as a word; and an initialism is an acronym that is pronounced in individual letters.