Initialism
Appearance
An initialism is an abbreviation formed by using the first letters, or initials, of a series of words, for example BBC or IBM.
In some cases, an initialism has been turned into a name because the meaning of the initials has been superseded. The YMCA is now just a name and is no longer short for Young Men's Christian Association. Likewise the name of the SAT college entrance test, that is, "the College Boards", does not stands for anything.
An abbreviation which forms a pronounceable word, such as NATO is an acronym.