now that's a tricky question indeed
osho was full of bullshit there - he maintained his enlightenment was 'once and for all', ie past which there is no other. i think that was a sales pitch.
enlightenment also means a time in history that historians have never forgotten - beginning of the modern etc.
i have my own views - i use it to refer to a profound realisation - the more substantial and devastating, the deeper the enlightenment. these are major turning points in one's life, where we wake up to something that changes us for the rest of our life.
my first was when a koan broke my mental frame society gave me, and I knew what i was going to do for the rest of my life, and that has never changed since. my second enlightenment came when i discovered myself talking and crying to a bush. these things just come to those who keep their feet on the path.