a polycrystal

Break a single crystal into countless tiny crystals, jumble their orientations, and pack them tightly together so they fill space with no gaps, and that is a polycrystal. Almost every metal you handle (a spoon, a coin, a car body panel) is polycrystalline: a mosaic of millions of tiny crystals, called grains, each with the same atomic pattern but pointing in a different direction. Think of floor tiles of one design laid at random angles.

Each grain is a small single crystal; where two grains meet, their patterns mismatch and the atoms sit in a thin, disordered seam called a grain boundary, like the ragged join between two tile patches laid at different angles. Grain size ranges from nanometers to millimeters. Because the grains point every which way, the material as a whole averages out to nearly the same properties in all directions (roughly isotropic), even though each individual grain is anisotropic.

Grain boundaries are central to how metals behave. They block the motion of dislocations, so a fine-grained metal is stronger than a coarse-grained one, the Hall-Petch effect, one of the most-used tricks in metallurgy. Boundaries also give atoms easy paths to diffuse and places for cracks or corrosion to start. Caveat: making grains smaller usually strengthens a metal at room temperature, but at high temperature abundant grain boundaries can let grains slide and actually weaken it, the benefit is not universal.