As Edwin Ashworth points out in a comment beneath the posted question, Wikipedia has a very useful discussion of this question as part of its broader coverage of the topic "Acronym":
An acronym is a word or name formed from the initial components of a longer name or phrase. Acronyms are usually formed from the initial letters of words, as in NATO (North Atlantic Treaty Organization), but sometimes use syllables, as in Benelux (short for Belgium, the Netherlands, and Luxembourg). They can also be a mixture, as in radar (Radio Detection And Ranging).
Acronyms can be pronounced as words, like NASA and UNESCO; as individual letters, like FBI, TNT, and ATM; or as both letters and words, like JPEG (pronounced JAY-peg) and IUPAC. Some are not universally pronounced one way or the other and it depends on the speaker's preference or the context in which it is being used, such as SQL (either "sequel" or "ess-cue-el").
There is no special term for abbreviations whose pronunciation involves the combination of letter names with words, or with word-like pronunciations of strings of letters, such as JPEG (/ˈdʒeɪpɛɡ/) and MS-DOS (/ˌɛmɛsˈdɒs/).
Having worked for many years in various publishing environments where editors and others were particular about distinguishing between terms that everyone agreed were acronyms (like NATO and NASA) and terms that everyone agreed were initialisms (like CIA and FBI), I can confirm that there is no widely accepted term for terms that fall somewhere in between (like JPEG and MCAT).
Neither acronym nor initialism is a particularly old word in English: Merriam-Webster's Eleventh Collegiate Dictionary (2003) dates former to 1943 and the latter to 1899, although a quick survey of early instances of initialism suggests that in the decades before acronym emerged it may have been narrowly used to refer to a signature consisting exclusively of the first letters of a person's name (such as "H.D." for "Hilda Doolittle").
In any event, as this Ngram chart for "acronym" (blue line) versus "initialism" (red line) indicates, —acronym is by far the more widely used term today and has been for at least the past sixty years:
In part that's because the current distinction in publishing between acronym (as letters pronounced as a single word) and initialism (as letters pronounced as letters) goes back only about fifty years. But the other reason is that the distinction is rarely important, in publishing or anywhere else.
If you want to explain to writers that they must provide the full spelling of an acronym or initialism the first time they use it and include the short form in parentheses after it, there is no practical reason to break the subject of the guideline into two terms on the basis of how the short form is pronounced. The guideline could say this:
Spell out any acronym or initialism on first occurrence, followed by the short form of the term:
North Atlantic Treaty Organization (NATO)
Central Intelligence Agency (CIA)
But the pronunciation issue is trivial to the instruction, and you might as well use the generic term (which is acronym) for both specific kinds of shortening (and for others that fall between them or slightly outside them, such as terms built from the first syllable or syllable of multiple words). The lack of an agreed-upon term for mixed-pronunciation short forms such as JPEG and MCAT likely reflects this practical consideration.
The only area where distinguishing between acronyms pronounced as words and acronyms (that is, initialisms) pronounced as individual letters might be useful in an editorial context relates to which indefinite article the term should take (as in the case of "an URL" versus "a URL" or "a FAQ" versus "an FAQ").