What did Hi-Fi stand for?

I'm curious, could you please clarify for me what the acronym "Hi-Fi" stands for? I've heard it used in various contexts related to audio equipment and sound quality, but I'm not entirely sure of its origins or meaning. I'm sure it's a term that many enthusiasts are familiar with, but for those who might not be as well-versed in the world of audio, an explanation would be greatly appreciated.