A mix of two kosher ingredients that can make it kosher or not-kosher depends on if it is “Batel B’Shishim” or nullified in 60 (Hebrew: בטל בשישים), and can apply to:
- Milk and meat
- Fish and meat
- Kosher for Pesach and Chametz (hint: the nullified in 60 is never applied for Passover!)
How is it calculated? 1/60th of the total volume of the food.
I can’t write better than Rabbi Dovid Heber from Star-K kosher: