What is the term for solid matter with atoms arranged in a regular, repeating pattern?

The term "crystal" refers to solid matter where atoms are arranged in a regular, repeating pattern, forming a structured lattice. This arrangement contributes to the unique physical properties of crystals, such as their specific shapes, cleavage patterns, and optical characteristics. Crystals can be found in various materials, including minerals and gemstones, where their orderly internal structure allows for consistent characteristics and behaviors.

Amorphous solids, in contrast, lack this regular atomic arrangement, resulting in different properties, while polymers are large molecules composed of repeating structural units, but they do not necessarily form repeating patterns at the atomic level like crystals do. Composites are materials made from two or more constituent materials with significantly different physical or chemical properties, which do not involve a repeating atomic structure. Thus, the defining feature of a crystal's regular and repeating atomic arrangement makes it the correct answer.