said of a mark, stain, etc: incapable of being removed or erased
said of ink: designed to make marks that cannot be removed or erased
said e.g. of a memory: not able to be forgotten
indelibility /-ʹbiliti/ noun
indelibly adv
[medieval Latin indelibilis, alteration of Latin indelebilis, from in-1 + delēre: see delete]