The answer to the crossword question is the word “MASS“, which refers to the “m” in the equation E=mc², famously formulated by physicist Albert Einstein. This equation describes the equivalence of mass and energy, stating that energy and mass are interchangeable and can be transformed into each other. The “m” in the equation stands for mass, which is a fundamental physical property of matter that measures the amount of matter in an object. In other words, mass is the amount of matter that an object contains, and it is a key parameter in understanding the behavior of objects in the physical world. Overall, the answer to the crossword question is “MASS“, a important term in physics and science that is central to our understanding of the universe.