Is 'invar' a class of materials, a specific material, or both? From the OP:
> There is, however, a class of metal alloys called Invars (think invariable), that stubbornly refuse to change in size and density over a large range of temperatures.
It's a small family of the nickel alloys. Lots of iron, as the primary alloying element, then other things. Different invar alloys exist, with different goals of 'invariant'. None are truly invariant, but some will behave better in a thermal band than others. Changing the alloying can get you into kovars (covariant, typically to glass or ceramic) another family. Inconel is yet another family of nickel alloy, favoured for its strength under some conditions. Small percentage alloying changes can have a sizeable impact.
Yes, it is a name, but no, is not THE name. Invar and Inconel are completely different alloys with completely different use cases. I have specified them both. In aerospace, Inconel 625 and 718 are used in high temperature structural and transport element applications in aircraft. Invar is never used in aircraft structure, but it is used to make tooling that needs to be invariant in size, especially composite layup mandrels that are cured in autoclave ovens.
> There is, however, a class of metal alloys called Invars (think invariable), that stubbornly refuse to change in size and density over a large range of temperatures.