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- Jaccard index, similarity coefficient, distance – Paul Jaccard
- Jahn–Teller effect – Hermann Arthur Jahn and Edward Teller
- Jaro–Winkler distance – Matthew A. Jaro and William E. Winkler
- Jarque–Bera test – Carlos M. Jarque and Anil K. Bera
- Johnson–Nyquist noise – John B. Johnson and Harry Nyquist
- Jordan's rule/law – David Starr Jordan
- Josephson constant, effect, junction – Brian David Josephson
- Joule's law (a.k.a. Joule–Lenz law) – James Prescott Joule and Heinrich Friedrich Emil Lenz
- Joule–Thomson effect (a.k.a. Joule–Kelvin effect) – James Prescott Joule and William Thomson, 1st Baron Kelvin
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