Full Idea
Atomic fact [Sachverhalt] is what corresponds to an elementary proposition [Elementarsatz] if it is true.
Gist of Idea
Atomic facts correspond to true elementary propositions
Source
Ludwig Wittgenstein (Letters to Russell [1919], CL 125)
Book Reference
Morris,Michael: 'Guidebook to Wittgenstein's Tractatus' [Routledge 2008], p.32
A Reaction
This is perhaps the key to the Tractatus, because it is the binding point between world and language. A true realist would allow for atomic facts that may go beyond even possible propositions.