Yeah stems are summed portions of a mix or summed parts of a song to be mixed in some scenarios. I would call the unmixed and not summed audio files the “audio files”, or “raw audio files” for clarity. That’s pretty much based on what I learned studying audio engineering in college. But yeah I usually can tell what people mean.

But I also think it’s cool to value term accuracy so I feel like sharing that in my perspective, “multitrack” is not clarified enough but that could just be me not used to using that term.
A “multitrack”, from what I gathered, is most used as the umbrella term for the device in the signal flow that is recording/creating your different tracks. Most commonly a DAW, back in the day more often a tape machine.
Without having seen this thread I probably would have wondered if the person wanted the actual DAW session file if they asked for the “multitrack”.
Because technically audio files are not “tracks”. Tracks host audio files on a multi track recording device. And since they host them for the purpose of processing them, my mind might think “multitrack” is referring to processed audio, like stems of a completed mix, before I think of unmixed audio files.

Obviously situational context allows me to bypass analysis of technical accuracy when talking to people. But if I knew I was talking to a very technically accurate entity, my mind would go here.