An initialism or alphabetism is similar to an acronym, a word in which each letter stands for another word. However, in an initialism, each letter is pronounced separately as if the word were being spelled. A commonly heard example is CEO, which stands for "chief executive officer." CEO is an initialism, rather than an acronym, because it is pronounced cee-ee-oh rather than cee-oh.
Originally, the word initialism referred to any abbreviation formed of initials, regardless of pronunciation. The term acronym, which is now more widely used, was coined by Bell Laboratories in 1943. Though initialism is an older word, attested from 1899 according to the Oxford English Dictionary, it did not come into widespread usage until the 1960s. Many people use these terms interchangeably, especially for initialisms that may be pronounced either letter by letter or as a word, for example, IRA, pronounced eye-ar-ay or eye-ra.
Though initialisms existed even in the ancient world -- for example, SPQR for Senatus Populusque Romanus, the official title of the Roman Empire --they have become increasingly common in recent decades. The proliferation of complicated terminology, especially in the fields of science and technology, have contributed to the convenience of acronyms and initialisms in the modern world. Internet slang and the popularity of texting have also added a great number of initialisms to the lexicon.
Conventions for writing initialisms vary among style guides and individual usage preferences. Originally, each letter was followed by a period, but many style guides now claim such punctuation is redundant, as the ellipsis is understood. Others prescribe using periods after each letter in an initialism, but not in an acronym (e.g. "F.B.I." vs. "NATO"). Most conventions use all capitals for initialisms and acronyms, though some acronyms may have only the first letter capitalized (e.g. "Nato"). Acronyms that have been linguistically incorporated as regular words, such as scuba and laser, are not capitalized.