Theory of relativity is a combination of two different theories or says two different combinations. First, one is Einstein’s special relativity theory and second one is general relativity theory. Concepts included in theory of relativity consider space-time as a single unified entity. According to concepts of theory of relativity, following are the elements of unified entity of space-time:Space and time (of course)
Relativity of simultaneity
Kinematic
Dilation gravitational time
Length contraction