Substances whose atoms or molecules are arranged in a definite pattern.

Hint : Try to recall the substances which have a definite pattern and their molecules or atoms are arranged in a fixed shape. By this, we remember that the solids have a definite shape but they do tend to change under pressure. So, the solid that doesn’t change after pressure will have a definite shape.

Complete Step By Step Answer:
We have to state a solid substance that has a definite pattern and arranged atoms. Such a substance with a specific predictable arrangement of molecules is known as Crystalline solids. They aren’t haphazard but are quite delicate and specific in the pattern. They are also referred to as ‘true solids’.
Crystalline solids are classified based on the nature of forces that hold the particles together. Metals and ionic compounds are included under the category of crystalline solids.
The atoms present in the crystal form a repeating pattern that is called the crystal lattice. Allotropes of carbon form crystalline solids. These are incompressible into smaller shapes. Because of the crystal lattice, the bonds in the structure have equal strength. The crystalline structure has a specific melting point because all the bonds have the same strength. So applying heat will disrupt all the bonds at the same time.
One great example of crystalline solids is Diamond.
This crystalline substance is made up of pure carbon atoms. Similarly, graphite also contains only carbon atoms. The only difference between the two is that they look very different. When the carbon atoms arrange themselves in a definite pattern the structure becomes very hard. This is why diamond is the hardest substance.

Note :
Crystalline solids are classified into 4 types: Ionic solids, metallic solids, covalent solids, and molecular solids. These crystalline solids show anisotropy. Their properties like conductivity, refractive index, or tensile strength change depending on where the force is applied.