Muggle Studies?

Okay, so I’m stuck in a slightly different place than the others who have asked for help here.

I have the magic word and most of the ingredients to cast the spell, but can’t seem to make head nor tail of “eternity.” What in the world does it mean by that? Is there a symbol or something named eternity? And how do I get it?

Thank you!

To the best of my recollection,

‘eternity’ is the Celtic-knotwork thing you got from the owls puzzle.