Archaic KJV Word
Heaven
Modern equivalent: afterlife destination
What Was Lost
The overlap. 'Thy will be done on earth as it is in heaven' was not a prayer about the afterlife but a prayer for heaven's reality to invade earth now. Heaven and earth were designed to overlap -- the temple was the overlap point. The biblical hope was not 'going to heaven' but 'heaven coming to earth.'
Closest Survivor in Modern English
heaven (still used but understood as 'where we go when we die' rather than 'God's dimension breaking into ours')
Peak Usage (1611)
KJV Genesis 1:1 -- 'God created the heaven and the earth'; Matthew 6:10 -- 'Thy kingdom come, Thy will be done in earth, as it is in heaven'
Died still used but relocated (~1800)
Hebrew shamayim ('the sky/God's dimension/the realm where God's will is done') was relocated from an overlapping dimension to a distant location where people go after death. Heaven became a place you go rather than a reality that comes here.
What Replaced It
“afterlife”
Post-death destination; shamayim was the parallel dimension of God's rule that overlaps with earth
“paradise”
Leisure destination; heaven was where God's will was perfectly done -- a place of purpose, not retirement
“the sky”
Physical atmosphere; shamayim was both literal sky and God's dimension simultaneously