Both acronyms and abbreviations are children of the same process — the abbreviation, which consists of reducing the size of words or expressions used with great frequency, in order to speed up writing. In principle, the distinction between them is as follows: the abbreviation uses a segment of the word, which is interrupted at a certain point, usually right after a consonant: ap. , from “apartment”, prof. , from “teacher”, min from “minute”, and so on. There are some “discontinuous” abbreviations, but they are in the minority, like dr., which uses the first and last letter of the word; in this case, the dot has the very important function of indicating that we are dealing with a reduced word. The acronym is formed by the initial letters of the words that form an expression (sometimes with some vowel interpolation, to improve the phonological profile): AIDS, for example, is a typical acronym, as its letters are the initials of the name of the disease (in English, “the acquired immunodeficiency syndrome ”). Also the popular CD (“ compact disc ”), as well as CD-ROM (“ compact disc reading only memory ”), in addition to DNA, ONU, MEC, FIESP and, apparently, your ATP (formed by the English initials — again! — “a denosine tri-phosphate ”). Now, don't worry: few people make a distinction between these two types of reduction; most of the dictionaries and spelling guides I know present, quite simply, a list of the “abbreviations and acronyms used in this work” — thus, without stress, without separating one from the other.