Final answer:
A crystal lattice is the three-dimensional arrangement of a crystal, with atomic patterns repeating regularly in three dimensions; the unit cell is the smallest repeating unit of this lattice.
Step-by-step explanation:
The subject of this question is the internal pattern controlling crystal shape, specifically how atomic patterns repeat regularly in three dimensions in a substance. A crystal lattice refers to this orderly, repeating three-dimensional arrangement.
Within the lattice, a unit cell is the smallest portion that displays this repeating pattern, which can be extended in all three dimensions to form the entire crystal. This concept is critical in understanding various properties of materials, including how they diffract X-rays, as each crystal may contain multiple families of planes that create complex interference patterns observable in experiments such as X-ray diffraction.