What is solid?
Solid is one of the four fundamental states of matter (the others being liquid, gas, and plasma). It is characterized by structural rigidity and resistance to changes of shape or volume. Unlike a liquid, a solid object does not flow to take on the shape of its container, nor does it expand to fill the entire volume available to it like a gas does. The atoms in a solid are tightly bound to each other, either in a regular geometric lattice (crystalline solids, which include metals and ordinary ice) or irregularly (an amorphous solid such as common window glass).
Solid materials may be three types according to order region:
Amorphous: are the materials having order only within a few atomic or molecular dimensions.
Polycrystalline: are the materials having some order regions called crystalline grains. The grains can be more or less aligned w.r.t. each other. Where they meet is called grain boundary.
Single-crystal: are the materials having a high degree of order or regular geometric periodicity throughout the entire volume of the material.