As I explained in an other article here on HN, another meaning (and the most important one imo) is the one Einstein actually wrote about: the inertia of a parti

As I explained in an other article here on HN, another meaning (and the most important one imo) is the one Einstein actually wrote about: the inertia of a particle depends on its energy. The thing is, you don't read E = mc² but rather (as Einstein wrote in his surprisingly easy to understand 2-pages paper [0]), m = E / c². The direct interpretation of this formulation is that inertial mass is actually just a side effect of the energy of a particle. Put into a catchy phrase, mass is energy at rest. Edit : [0]: https://www.fourmilab.ch/etexts/einstein/E_mc2/e_mc2.pdf https://www.fourmilab.ch/etexts/einstein/E_mc2/e_mc2.pdf, translated in english. Note the understatement of the sentence before the last one: "It is not impossible that with bodies whose energy-content is variable to a high degree (e.g. with radium salts) the theory may be successfully put to the test."