eternal
adjective /ɪˈtɜːnl/
  /ɪˈtɜːrnl/
Idioms - without an end; existing or continuing forever- the promise of eternal life in heaven
- She's an eternal optimist (= she always expects that the best will happen).
- eternal truths (= ideas that are always true and never change)
- You have my eternal gratitude (= very great gratitude).
- To his eternal credit (= very great credit), he never gave up.
- Newspapers are simply responding to the eternal fascination of their readers with the private lives of the rich and famous.
 Extra Examples- The price of freedom is eternal vigilance.
- The story is about a woman who is offered the elixir of eternal youth.
- The two heroes swore eternal friendship.
- The universe was about to be plunged again into eternal darkness.
 
- [only before noun] (disapproving) happening often and seeming never to stop synonym constant- I'm tired of your eternal arguments.
 
Word Originlate Middle English: via Old French from late Latin aeternalis, from Latin aeternus, from aevum ‘age’.
Idioms 
hope springs eternal 
- (saying) people never stop hoping