[yongjik]

Well, maybe, but I think you're mixing the everyday definition of "relativity" (i.e., things are relative) with what physicists mean by "relativity" (i.e., the Theory of Special or General Relativity, both theorized by Einstein).

[random314]

Einstein's theory of relativity is exactly what you are referring to as "everyday definition of relativity ".

[tux1968]

> but I think you're mixing the everyday definition of "relativity"

Well, yes.. because Einstein's theory beautifully makes all the math work out, explaining one interpretation of motion as the real one. But what it did at the same time was show that there is no "fixed grid" of space independent of the objects themselves. Which is what leads to my uneasy feeling of how we ever can say one interpretation is more real than another -- i admit it may just be a nonsensical perspective.

As for what all physicists mean by relativity in general... I can highly recommend this old series from the National Science Foundation:

https://www.youtube.com/watch?v=Y75kEf8xLxI