In my opinion a solid is defined as a system of atoms and electrons that are tightly bound by interactive forces.
If the forces are weaker, the geometry changes and we’d get a different state.
The interactions are between the nuclei and delocalised electrons, between electrons and between nuclei. I think what also makes a solid what it is, are its rigid layers, they don’t Slide Over one another and atoms don’t flow which means, there is a certain kind on continuity in it.