A citation of Emily Dickinson's that I read today went straight into my bones, or rather, made me feel like my bones are hollow and that in them the whispers and fluid starlight of poetry resides:
"If I read a book and it makes my whole body so cold no fire ever can warm me I know that is poetry. If I feel physically as if the top of my head were taken off, I know that is poetry."
Sadly, I do not know where this is collected from, but it is probably from a letter, as we have no other prose from her hand as far as I know. The art of describing poetry - or should I call it the difficulty, the neverending trial of describing poetry, results in many poetic sayings which point directly to the unspeakable, unnamable thing that is poetry itself.
Just in these two sentences, her strange genius is apparent - or rather, her openness to some unknown and unending realm of reality. I don't know a better way of describing the access to poetry - both creating and reading - than to have the daring to look the unknown straight in the eye. There might always be an abyss ahead (probably there is), but there is no saying that abyss won't be filled with startling and altering newness, ready to greet us.
If I should add an entrance to poetry myself, it would in this moment sound like this:
When light and weight, stone and stellar space meet in me and contains a whole new field, I know that is poetry. When light gathers in me and wishes to burst every boundary, I know that is poetry.
...which is rather more ethereal than Emily's quite corporeal and solid way of describing these things poetic. But pointing, I hope.