Share your favorite physics equation and what it describes

[AstrophysicsX]

Post your favorite physics equation. It can be about anything just make sure you explain what the equation describes and who formulated it.

 

[Schreiberdk]

Mine is probably the Dirac Equation, which is a relativistic generalization of the Schrödinger Equation. The solutions to the equation described the wavefunction of a particle with relativistic corrections.

It is my favorite equation, because it is the foundation of quantum field theory (relativitic quantum mechanics).

Link: http://en.wikipedia.org/wiki/Dirac_equation#Mathematical_formulation