A repeating arrangement of points that completely fills up space - Lattice vector - line connecting any two points in the lattice - unit vector = smallest lattice vector - Any lattice vector can be defined by linear combination of unit vectors # Types ## 2D 1. Square 2. Rectangle 3. Hexagonal 4. Oblique ## 3D 1. Triclinic 2. Monoclinc 3. Orthorhombic 4. Tetragonal 5. Cubic 6. Trigonal 7. Hexagonal [[Miller Indices]]