Paul Ehrenfest

“ A body is relative-rigid, that is: it is deformed continuously at any movement, so that (for a stationary observer) each of its infinitesimal elements at any moment has just that Lorentz contraction (compared to the state of rest) which corresponds to the instantaneous velocity of the element's center. ”

“ Born has based this definition – in accordance to the basic idea of relativity theory – not on the system of measurement of a stationary observer, but on the (Minkowskian) measure-determinations of, say, a continuum of infinitesimal observers who travel along with the points of the non-uniformly moving body: for each of them in their measure the infinitesimal neighborhood should appear permanently undeformed. ”