The question "what is relativity?" is best answered by Albert Einstein himself (TIME Magazine's "person of the century"). "So Professor Einstein, if you would bear with us, what is relativity?"
In his essays: "The Theory of Relativity and other Essays" 1996, Citidel Press, Einstein "answered" as follows:
"The theory of relativity is that physical theory which is based on a consistent physical interpretation of the concepts of motion, space and time.
"The name `theory of relativity' is connected with the fact that motion from the point of view of possible experience always appears as the relative motion of one object with respect to another.
"Motion is never observable as 'motion with respect to space' or, as it has been expressed, as 'absolute motion'.
"The 'principle of relativity' in it's widest sense is contained in the statement: The totality of physical phenomena is of such a character that it gives no basis for the introduction of the concept of 'absolute motion'; or shorter but less precise: There is no absolute motion.
"The development of the theory of relativity proceeded in two steps, 'special theory of relativity' and 'general theory of relativity'. The latter presumes the validity of the former as a limiting case and is its consistent continuation."
What Einstein meant with the last paragraph above is that his general theory of relativity includes his special theory of relativity in certain limiting, or special cases. Stated differently, general relativity reduces to special relativity in the case of negligible gravity.
To discover how we as engineers (or other interested people) may interpret Einstein's very brief "answer", here are two links that tell the story:
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