≍ means equivalent to, in math and logic.

You've perhaps seen the meaning written just as a tilde "~" such in ASCII-based text, or written as "==" in JavaScript meaning loosely equal to. IIRC the symbol is also useful for asymptotes and limits, such as Big O notations.

ChatGPT says it is not common outside of specialized mathematics

I would have expected '≈' as the symbol.

Right, that's normally what I'd use in this situation. But that means something more like "approximately equal to", whereas this notation seems more akin to "equivalence" as in an equivalence class. I don't have a clear memory of what notation we used for that in school, but it wasn't this symbol.

I thought that was “approximately equal to”.

In hydraulics that symbol is a fixed flow restrictor such as an orifice plate

Thanks! I just followed what GNOME Characters suggested.