Not sure what you mean by 'esoteric' or more 'concrete'
It's a spiritual term which comes from spiritual practices. The phenomenon itself is obviously spiritual ie: esoteric the result of which is very concrete. If you take historical accounts of zen masters for example then you could say that after contemplating a zen koan given to you personally by one such master then whilst doing very concrete in life things you may notice something and become enlightened.
Apparently that wasn't such an unusual thing in some parts of the world once upon a time.
Personally I prefer the term path towards.
How does one know when they have reached enlightenment?
One knows when they have achieved understanding to do with riding a bicycle. No more resistance, fear, confusion, trepidation, blame, regret etc. etc. just oneness and ability and enjoyment with a bicycle. It took theory (data, subjective) and practice (concrete action) to achieve.
Lots of mini enlightenment's
This is what I was driving at, of course enlightenment is a spiritual term and so in that respect is a result of spiritual practices but I just wanted to know whether it had tangible manifestations i.e. my use of the term concrete. Sorry if I wasn't clear with my meaning but you seem to have understood based on your example of zen masters and the analogy of riding a bicycle.