Definition #3: Any literary composition written with an intensity or beauty of language more characteristic of poetry than of prose: a prose poem.
Putting that in my own words for my own use: anything written with intense, condensed, beautiful language.
Well known examples:
- Lincoln's Gettysburg Address -- its simplicity, clarity and brevity are not emulated enough in modern oratory (and that's about the biggest understatement I ever made!)
- Preamble to the US Declaration of Independence -- "When in the course of human events..." sounds more like the opening of a Shakespeare sonnet than the beginning of the primary political document founding a great nation.
- Patrick Henry's fiery "liberty or death" speech -- beats like a wardrum, "I know not what course others may take, but as for me..."
- Rev. Martin Luther King Jr's "I Have a Dream" speech is also beautiful for its powerful and simple message.
Now those are some poems that caused something!