In condensed matter physics, an amorphous (from the Greek a, without, morphé, shape, form) or non-crystalline solid is a solid that lacks the long-range order characteristic of a crystal.

In part of the older literature, the term has been used synonymously with glass. Nowadays, "amorphous solid" is considered to be the overarching concept, and "glass" the more special case: A glass is an amorphous solid that transforms into a liquid upon heating through the glass transition.

Other types of amorphous solids include gels, thin films, and nanostructured materials.

Even amorphous materials have some short-range order at the atomic length scale due to the nature of chemical bonding. Furthermore, in very small crystals a large fraction of the atoms are located at or near the surface of the crystal; relaxation of the surface and interfacial effects distort the atomic positions, decreasing the structural order. Amorphous form have higher dissolution rate compare to any other form.Even the most advanced structural characterization techniques, such as x-ray diffraction and transmission electron microscopy, have difficulty in distinguishing between amorphous and crystalline structures on these length scales.^{[citation needed]}