Relativistic mechanics refers to any form of mechanics compatible with either theories of Special Relativity

Special relativity

Special relativity is the physical theory of measurement in an inertial frame of reference proposed in 1905 by Albert Einstein in the paper "On the Electrodynamics of Moving Bodies".It generalizes Galileo's...

 and General Relativity

General relativity

General relativity or the general theory of relativity is the geometric theory of gravitation published by Albert Einstein in 1916. It is the current description of gravitation in modern physics...

. It is a non-quantum mechanics of a system of particles or of a fluid in cases where some of the velocities are comparable to the speed of light.

Special relativity states that motion is relative and the laws of physics are the same for all experimenters irrespective of their reference frames.