Einstein

Physicists probably rarely use the work "geinus" in describing a colleague, but many have used that word when describing Einstein. His work was VERY IMPORTANT to physics, and his theories (mostly confirmed by experiment) are vital to modern physics.

His most important contributions:

• The theory of special relativity. It changed our conception of space and time. Without it we could not construct the particle accelerators that are used nowadays to probe the small-scale world of sub-atomic particles. All other theories must be consistent with the basic concepts of special relativity (basically, that physics must be the same for any observers who only differ by their uniform --- straight line, constant speed --- motion with respect to each other). If the equations that a physicist invents to explain some part of the world predicts results that depend upon the motion of the observer (experimenter) then the new equations (the new theory) must be wrong. 
   
• His introduction of the idea that light comes in particles called photons was one of the foundation pieces upon our quantum mechanical view of the world was built (e.g., the explanations of the workings of the atom and smaller scale structures). He received the nobel prize for this work. 
   
• His theory of general relativity. The modern theory of gravity. It is a relativistically correct theory (Newton's is not), and has been mostly verified to be correct, however, it has not been thoroughly tested since complete tests would require the manipulation (or direct observation) of objects containing huge masses. This is the theory that predicts the existence of black holes --- objects that are used in many explanations of bizarre phenomena observed in the universe (e.g., quasars).
  

After completing work on general relativity, Einstein tried to construct a relativistically correct theory which explained gravity and the electric and magnetic forces as different aspects of the same force (or same set of equations). It had already been shown in the 19th century that electric and magnetic forces were different aspects of the same phenomena, so there was reason to believe that might be true of all forces. Einstein never succeeded. Since then, it has been shown that the electric and magnetic forces, and so-called "weak nuclear force" are different aspects of the same force, but nobody has successfully united the "strong nuclear force" (now called "quantum chromodynamics"), or gravity, with the electric/magnetic/weak force (called the "electroweak force").