I think it's called semantic disagreement. I have a background in philosophy, and I suspect a lot of disagreements are the result of disagreements over the meanings of a word, and the fact that we are using words differently is so very easily missed when we do disagree.

For example, we can argue endlessly over whether esports are 'sport', but it's really quite simple. You tell me what you mean by 'sport', and then I'll tell you whether or not esports are a sport.

In some cases, it might be a little weirder. For example, because of the discovery of the nature of visible light, 'light' now has more than one meaning attached, and these meanings are incompatible. If it's dark in a room, we might say "there is no light", but on the other hand it could be bathed in EM waves outside the visible spectrum (such as the cosmic background radiation), in which case it's full of 'light'. Light has these two meanings: one to do with whether I can see or not, and the other to do with EM waves. Context can help determine which meaning is being used.

As we grow and learn and communicate, we pick up how words are used by other people, and we try to match our usage to the way everyone else uses these word. But sometimes we don't pick up quite the same meanings even though there's significant overlap (semantic extension). There lies confusion and a great deal of pointless disagreement. I think it's mostly pointless (from a philosophical perspective) to debate about what a word should mean. If there's disagreement, stipulate how you're going to use words, or invent new ones for the conversation, and move on.

In high school debate, it was referred to as "grounds"- essentially, laying out definitions for terms important or relevant to the topic established the terrain, to use a battle metaphor.

If you're using the same words but with different beings, it's like two opposing forces not being on the same field of battle.

Two ships passing in the night is another metaphor, and is probably more common in English as a phrase.

Sounds like a version of what I've heard called "talking past eachother".

Semantic mismatch?

Miscontextualization?

I like the suggestions, I do think it need to be a bit shorter to be viral. Riffing on this, how does "we've misdefined this" sound?

Why not just “let’s define ours terms so we don’t talk passed each other? What do you mean by X?”

#letsdefineourtermssowedonttalkpassedeachother?whatdoyoumeanbyx? is sorta hard to type out, a shorthand term may lend itself to a common understanding of the situation. Like "a picture of yourself" got coined as a selfie, it seems obvious now through constant use, but there was a time when the phenomenon existed and we had a lexical gap in describing it.

“It’s semantics” is the briefest way I usually describe this phenomenon.