What shape is this?

As a German, I can offer “Ein Schluck Wasser in der Kurve”.

Void

Cat

Either good kitten or sausage

I think it’s one of these…

Reticulated sp(l)ine.

Ah, the classic Egyptian Eye pose.

Actually there is unironically a word for it:

https://en.wikipedia.org/wiki/Opisthotonus

Catenary curve.

Purrabola

Hoooogarfield curve.

Catatellogram.

~

That shape is called a “cat”.

Cat hat hook.