For what it's worth, I recall the CSS sprite technique being referred to as image mapping too.

Sure, it's different from the <map> element (which is officially an image map, as you say).

But it's effectively mapping out the sections of a large image that you care about and then using CSS to display parts of it

I'm actually conflating both, you're right. I'm thinking of sprite-ed CSS, but also the really really old style imagemap where the same image could be clickable in multiple areas for different purposes.

I'm sure it's due to lack of standardization on the early web, but the really wild thing to me is that there are both client-side and server-side image maps (both still specified in HTML5). With the former, the various regions of the image are defined within the HTML and more or less act like regular links. With the latter, the client sends the coordinates to the server and gets redirected, so the user has no idea which pixels correspond to which URL.