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Polycrystalline refers to a solid material composed of many small crystals, known as crystallites or grains. These grains are randomly oriented with respect to each other, and their boundaries form a network throughout the material. In contrast, a monocrystalline material consists of a single, continuous crystal lattice.

The properties of a polycrystalline material are often an average of the properties of its constituent grains and their grain boundaries. These properties can be significantly different from those of a single crystal of the same material. For instance, polycrystalline materials are generally less strong and less ductile than their monocrystalline counterparts due to the presence of grain boundaries, which can impede the movement of dislocations (defects in the crystal lattice).

Common examples of polycrystalline materials include most metals and ceramics used in everyday applications, such as steel, aluminum, and pottery. The size and distribution of the grains can be controlled during the manufacturing process, influencing the material's mechanical, electrical, and optical characteristics. For example, finer grain sizes typically lead to increased strength. Techniques like annealing can be used to alter the grain structure, sometimes leading to grain growth or recrystallization. The study of polycrystalline materials is crucial in materials science and engineering for understanding and optimizing material performance.