Full Idea
At age twenty, Von Neumann devised the formal definition of ordinal numbers that is used today: an ordinal number is the set of all smaller ordinal numbers.
Gist of Idea
Von Neumann defined ordinals as the set of all smaller ordinals
Source
report of John von Neumann (works [1935]) by William Poundstone - Prisoner's Dilemma 02 'Sturm'
Book Reference
Poundstone,William: 'Prisoner's Dilemma' [OUP 1992], p.29
A Reaction
I take this to be an example of an impredicative definition (not predicating something new), because it uses 'ordinal number' in the definition of ordinal number. I'm guessing the null set gets us started.