Emily Dickinson’s test of poetry:
If I read a book [and] it makes my whole body so cold no fire can ever 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. These are the only way I know. Is there any other way.
Dickinson gave this definition in 1870 to Thomas Wentworth Higginson. I found it in Edward Hirsch’s How to Read a Poem and Fall in Love with Poetry, which I am thoroughly enjoying. You can read the first chapter, including more on the quote, online thanks to the Poetry Foundation.