What does "original work" mean?

When the source of a proof or definition is given as "original work", it means that a contributor of "The Book of Statistical Proofs" came up with the proof or definition themselves. As outlined in the FAQ section of the Citing Conventions, it is perfectly OK to submit a proof or definition without source – provided it is mathematically sound and statistically correct. On the long run, it is however desirable that each proof and definition is given a source. You can help by contributing to the StatProofBook and providing a source for one of the Proofs without Source or Definitions without Source.