the lattice parameters
To describe the shape and size of the unit-cell box you need six numbers: three edge lengths and three angles. These six are the lattice parameters — the crystal's most basic dimensions, like reporting the length, width, height, and lean of a slanted box.
They are a, b, c (the edge lengths, typically just a few angstrom; 1 angstrom = 10^-10 m = 0.1 nm) and alpha, beta, gamma (the angles between the edges — alpha between b and c, beta between a and c, gamma between a and b). For example, copper is cubic with a = b = c = 3.615 angstrom and alpha = beta = gamma = 90 degrees, so a single number pins down the whole cell. The crystal system dictates which parameters must be equal or fixed at 90 or 120 degrees.
Lattice parameters are what diffraction measures first — peak positions fix the cell dimensions to four or five significant figures. They change with temperature (thermal expansion), pressure, and composition (Vegard's law in alloys), so tracking a tiny shift in a lattice parameter is a sensitive probe of strain, alloying, or a phase change.
Tetragonal white tin has a = b = 5.83 angstrom but c = 3.18 angstrom, with all angles 90 degrees — so it needs two independent lengths, not one. Cubic diamond needs just a = 3.567 angstrom. The crystal system fixes how many of the six parameters are truly free.
How many of the six numbers are independent depends on the crystal system.
Lattice parameters depend on temperature and pressure; a quoted value implicitly assumes standard conditions. Reporting a structure without stating the temperature can hide real differences.