Eternity (10)
'eternity' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for eon that I've seen before include "Vast age (alt. sp.)" , "Geological period" , "billion years" , "thousand million years" , "Very long period (US and technical spelling)" .)
'eternity' is the definition.
(thesaurus)
This is all the clue.
(Other definitions for perpetuity that I've seen before include "In this is forever" , "In(de)finite duration" .)