Bill,
Maybe I've been wrong all these years but I thought that defined in broad terms, a mnemonic is a device, procedure, or operation that is used to improve memory. Defined in narrow terms, and as you say is what is usually meant by the word — a mnemonic is a specific reconstruction of target content intended to tie new information more closely to the learner's existing knowledge base and, therefore, facilitate retrieval.
I thouhght acronyms were one of a variety of mnemonic techniques, including keywords, pegwords, acrostics, loci methods, spelling mnemonics, phonetic mnemonics, number-sound mnemonics, and Japanese “Yodai” methods.
I got some of these from a dictionary definition. I don't know about Yodai, I thought it was a little green George Lucas character
I guess I'm a Fat Acerbic RollingGreen Tightwad