What part of speech is HMM?

HMM (Hidden Markov Model) is not a single word, but an acronym. It stands for Hidden Markov Model, a statistical model used in various fields like speech recognition, natural language processing, and bioinformatics.

Acronyms themselves aren't typically classified as parts of speech because they function as nouns representing the full phrase they abbreviate.

So, in the context of "HMM," it acts as a noun referring to the "Hidden Markov Model."