Polycrystalline materials are solids that are composed of many crystallites of varying size and orientation. The variation in direction can be random (called random texture) or directed, possibly due to growth and processing conditions. Fiber texture is an example of the latter.
Almost all common metals, and many ceramics are polycrystalline. The crystallites are often referred to as grains, however, powder grains are a different context. Powder grains can themselves be composed of smaller polycrystalline grains.
Polycrystalline is the structure of a solid material that, when cooled, form crystallite grains at different points within it. Where these crystallite grains meet is known as grain boundaries.