An acronym is a word that's made up of bits of other words. At least, it is if you can say it.
For example, you can't really say BBC as a word, you can only say the letters: you say B - B - C, not bubucy. This means that BBC is more of an initialisation than an acronym. But people do argue about this.
Acronyms are mostly fairly new things, and the interesting ones are usually fakes. Cop, for example, does not stand for Constable On Patrol, but is someone who cops (arrests) people. Posh does not stand for Port Out Starboard Home (the best cabins to stay in if you're going East - or so the story goes) but probably comes from an old word for a dandy.
Scuba is a real acronym: Self-Contained Underwater Breathing Apparatus. That's quite a good one, but my favourite acronym by miles has to be laser.
Laser is made up of the first letters of Light Amplification [by] Stimulated Emission [of] Radiation: except that the scientists among you will know that a laser doesn't amplify light at all. It makes it oscillate.
So why on earth the scientists decided against using the acronym LOSER I've no idea.
Acronym is from the Greek word acros, topmost, and onym, which is from the Doric word onoma, name.
No, I can't see what topmost has got to do with acronyms, either.