an amorphous solid

Freeze a jostling crowd in a single instant: everyone stands close to a few neighbours, but there is no marching-band grid, no rank and file stretching to the horizon. An amorphous solid is like that. The word means without form (from Greek), and it describes a solid whose atoms have short-range order (each atom bonded to the right neighbours) but no long-range order (no pattern that repeats across the whole material). Ordinary window glass is the classic example.

How does a solid end up disordered? Usually you cool a liquid so fast the atoms cannot line up into a crystal before they lock in place, freezing in the jumbled liquid structure. In silica glass, SiO4 tetrahedra are still joined at their corners but point in random directions, unlike quartz, which is the very same chemistry crystallized into a neat repeating array. Many polymers are partly or fully amorphous because their long, tangled chains cannot easily fold into a regular grid.

The consequences are practical. With no crystal planes, glass has no cleavage directions, so it fractures in smooth curved shells rather than splitting along flat faces; it is usually isotropic (the same in every direction) and often transparent. Instead of a sharp melting point it has a glass-transition temperature Tg, where it gradually stiffens from a liquid to a rigid solid. Some metals can even be quenched into amorphous metallic glasses, which can be remarkably strong.