Initialism
An initialism is an abbreviation formed by using the first letters, or initials, of a series of words, for example "BBC", or "IBM". The term initialism is often used by those who make a sharp distinction between an initialism and an acronym; they reserve the term acronym for cases when the letters form a pronounceable word, like "NATO" or "AIDS", and use the term initialism when they do not, being pronounced instead by sounding out the name of each constituent letter. (Thus, by this definition, BBC ("Bee Bee See") is an initialism, while NATO ("Naytow") is an acronym.) Others do not make this distinction, and use the terms interchangeably (though the term acronym is used much more frequently in this case).
In some cases, an initialism has been turned into a name. The letters making up the name of the SAT college entrance test, for example, no longer officially stand for anything. This trend has been common with many companies hoping to retain their brand recognition while simultaneously moving away from what they saw as an outdated image: the company formerly named Kentucky Fried Chicken changed its name to "KFC" to avoid negative connotations of the word "Fried"; British Petroleum changed its name to "BP" to emphasize that it was no longer only an oil company (captured by the motto "beyond petroleum"); and Silicon Graphics, Incorporated changed its name to "sgi" to emphasize that it was no longer only a computer graphics company.
The official "full" form of an initialism -- what the initialism stands for -- may also be changed without revising the initialism. SADD, for instance, originally Students Against Drunk Driving, changed the full form of its name to Students Against Destructive Decisions. YM originally stood for Young Miss, and later Young & Modern, but now stands for simply Your Magazine.