The first thing that came up to my mind is the “school” definition of the solid: a state of the matter, where shape and volume are constant (in a small interval: the coefficient of thermal expansion is non-zero but by several orders of magnitude smaller than in liquids, not to speak of gases and plasma)

Then I have found a thermodynamic definition of solid: it is a lowest temperature phase of a matter (except of helium). From this point of view, amorphous matter is not a solid but a metastable liquid. That leads to a definition of solid through a symmetry: solids have a long-range order

Interesting that many people here define solid as a matter with strongest bonds between its particles (atoms, ions, molecules)