a crystalline solid
Picture stacking oranges neatly in a crate, or laying square tiles across a floor: the same little arrangement repeats over and over in every direction. A crystalline solid is a material whose atoms sit in an orderly, repeating three-dimensional pattern that continues, unbroken, across millions upon millions of atoms. Most metals, table salt, diamond, and most ceramics are crystalline.
The defining feature is long-range order: if you know where a few atoms are, you can predict where atoms sit a thousand atomic spacings away. The smallest repeating block is called the unit cell, and copies of it stacked in all three directions build the whole crystal. In table salt, sodium and chlorine ions alternate on a cubic grid, and that hidden regularity is exactly why a salt grain, under a lens, looks like a tiny cube.
Why care? The way atoms are arranged is the first level of structure that controls a material's properties: how it bends, conducts electricity, or transmits light. Because the repeating pattern is directional, a crystal's properties can depend on the direction you measure them (this is called anisotropy). Nearly all metals and many ceramics used in engineering are crystalline.
Table salt (NaCl): sodium and chlorine ions alternate on a cubic grid. That microscopic regularity is why salt grains, viewed under a lens, are tiny cubes with flat faces and right-angle corners.
The little cubes of a salt grain are the outward sign of a cubic lattice inside.
A material can look shiny and solid without being crystalline — window glass is a solid but is amorphous, not crystalline. And a smooth outer shape is not proof of an ordered inside.