What law did Isaac Newton primarily define?

Isaac Newton is primarily recognized for formulating the laws of motion and gravity, which are fundamental principles that describe the behavior of objects in motion and the force of gravity acting upon them. His three laws of motion detail how forces influence an object's movement, addressing concepts such as inertia, acceleration proportional to force, and action-reaction pairs. Additionally, Newton's law of universal gravitation articulates how every mass attracts every other mass in the universe, providing a comprehensive framework for understanding both terrestrial and celestial mechanics. This foundational work in physics paved the way for classical mechanics and significantly influenced countless scientific advancements thereafter.

In contrast, the other options pertain to different fields of science. The laws of genetics are primarily associated with figures like Gregor Mendel, thermodynamics is a branch of physics that deals with heat and energy transfer discovered by various scientists over time, and electromagnetism is traditionally linked to James Clerk Maxwell's equations.