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In condensed matter physics and materials science, an amorphous or non-crystalline solid is a solid that lacks the long-range order, which is a characteristic of a crystal. In some older articles and books, the term was used synonymously with glass. Today, however, "glassy solid" or "amorphous solid" is considered to be the overarching concept, and glass is considered to be a special case: glass is an amorphous solid maintained below its glass transition temperature. Polymers are often amorphous.
Amorphous materials have an internal structure comprising interconnected structural blocks that can be similar to the basic structural units found in the corresponding crystalline phase of the same compound. Whether a material is liquid or solid depends primarily on the connectivity between its elementary building blocks; solids are characterized by a high degree of connectivity whereas structural blocks in fluids have lower connectivity.
In the pharmaceutical industry, some amorphous drugs have been shown to offer higher bioavailability than their crystalline counterparts as a result of the higher solubility of the amorphous phase. However, certain compounds can undergo precipitation in their amorphous form in vivo, and can then decrease mutual bioavailability if administered together.