I'd say that, really, it's impossible to describe an orgasm.  Apart from anything else, there's a nuance of difference with every single one - a bit like eating a favourite dish - say chilli con carne - at once recognisably similar but each time you make it it's slightly different, even using the same ingredients.  

Anyhow, here's a poem which for me best encapsulates the moment of orgasm.


Rhetorical Questions
Hugo Williams

How do you think I feel
when you make me talk to you
and won't let me stop
till the words turn into a moan?
Do you think I mind
when you put your hand over my mouth
and tell me not to move
so you can "hear" it happening?

And how do you think I like it
when you tell me what to do
and your mouth opens
and you look straight through me?
Do you think I mind
when the blank expression comes
and you set off alone
down the hall of collapsing columns?

Aphra