Its original definition was the study of systems under the action of forces.
Today there are different types of mechanics which pertain to different systems.
The most familiar form of mechanics is Newtonian mechanics. This is a description of the physical principles underlying force, mass and motion that holds for all 'classical' systems. Classical systems are those where the scale of the system is too large to make quantum effects noticeable, and the velocity of the system is far less than that of light, making relativistic effects negligible. Thus all familiar systems are covered by Newtonian mechanics. Specializations within Newtonian mechanics include statics, dynamics, fluid mechanics, celestial mechanics and many others.