The objects which have a definite shape, distinct boundaries and a fixed volume is called as solid objects, or we say they are in solid-state.
A book, pen, pencil.
A car, bike, plan, bus, etc.
All the objects which have definite shape falls into the category of solid.
The solid objects tend to stay on their original shape despite the external force.
For example, if we apply external force to the piece of chalk, it will break into pieces, but the shape and structure of the particles will not be altered as solids are rigid bodies.
Remember that the molecules inside the solids are so condensed, and this is the reason that they are rigid and maintain the definite shape.
The intermolecular force in the solid is the strongest force compare to the liquid and gas. This strongest inter molecule force makes the solid matter denser and gives a certain shape and structure to it.
Observe the above figure. You can see that the molecules inside the solid are dense, and that's why solid objects are so rigid.