Initialism originally referred to abbreviations formed from initials, without reference to pronunciation, but during the middle portion of the twentieth century, when they saw more use than ever before, the word acronym was coined for abbreviations which are pronounced as a word, like NATO or AIDS.
In the English language, the widespread use of acronyms, initialisms, and contractions is a relatively new linguistic phenomenon, having become most popular in the 20th and 21st centuries.
When initialisms are defined in print, especially in the case of industry-specific jargon, the initial letters of the full words are often capitalized.
An initialism is an abbreviation formed by using the first letters, or initials, of a series of words, for example BBC, IBM, or NATO.
The term initialism is often used by those who prefer to define an acronym as an abbreviation whose letters form a pronounceable word, like NATO or AIDS.
"Initialism" is then the name of the category for abbreviations that don't meet the strict definition of acronym and are pronounced as a series of the names of the constituent letters.