Everlasting Ink

"Some had merely signed their names in Everlasting Ink; others had carved their initials into the wood; still others had left messages."

- Description of the messages left in the memorial sign at the Potters' Cottage

Everlasting Ink was a type of ink. Presumably, from its name, anything written using this type of ink was bewitched to last forever and never fade.

When Harry Potter observed the memorial sign at the Potters' Cottage in Godric's Hollow, visible only to wizards, he saw that a number of individuals had signed their names on the sign using this type of ink.

Appearances
Harry Potter and the Deathly Hallows