Lise Meitner helped discover nuclear fission and explained its process theoretically in 1939.
Meitner worked with Otto Hahn and Fritz Strassmann on the bombardment of uranium with neutrons. Hahn and Strassmann found evidence of barium, but Meitner and her nephew Otto Frisch correctly interpreted the result as the uranium nucleus splitting. Frisch coined the term “fission,” borrowing it from biology.
The explanation appeared in a 1939 publication by Meitner and Frisch. Their account showed that the split released a large amount of energy, consistent with Einstein’s mass–energy relationship. Meitner herself did not receive the 1944 Nobel Prize in Chemistry, which went to Hahn alone, a decision that remains controversial.
A common mix-up is calling Meitner the sole discoverer. The experimental evidence came from Hahn and Strassmann, while Meitner and Frisch supplied the crucial physical interpretation.