无定形体，或称非晶体、非晶形固体，是其中的原子不按照一定空间顺序排列的固体，与晶体相对应。常见的无定形体包括玻璃和很多高分子化合物如聚苯乙烯等。只要冷却速度足够快，任何液体都会过冷，生成无定形体。其中，原子尚未排好在热力学上有利的晶态中的晶格或骨架便已失去运动速度，但仍保留有液态时原子的大致分布。

由于熵的缘故，即使冷却速度很慢，很多聚合物仍会生成无定形体。

In condensed matter physics and materials science, 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 some older books, 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 exhibits a glass transition. Polymers are often amorphous. Other types of amorphous solids include gels, thin films, and nanostructured materials such as glass.