Archaic KJV Word
Eternal
Modern equivalent: forever
What Was Lost
The qualitative dimension. Jesus defined eternal life as knowing God -- not as existing for infinite time. Aionios life was God's own quality of existence shared with humans. A person could begin experiencing eternal life now, in the present, because it was not about duration but about the source and quality of the life.
Closest Survivor in Modern English
eternal (still used but heard as 'never-ending' rather than 'God-quality')
Peak Usage (1611)
KJV John 17:3 -- 'This is life eternal, that they might know thee the only true God'
Died still used but temporalized (~1800)
Hebrew olam ('hidden time/age-spanning/beyond the horizon') and Greek aionios ('pertaining to the age/quality of God's own life') were reduced to 'lasting forever' -- a merely quantitative concept of endless clock-time rather than qualitative divine existence.
What Replaced It
“forever”
Clock-time extension; aionios described a quality of life -- God's own life-quality, not just long duration
“everlasting”
Still durational; eternal life in John 17:3 was defined as knowing God, not as living endlessly
“permanent”
Static and unchanging; olam was dynamic -- the hidden, unfolding purpose of God across ages