"In atomic physics, the fine structure describes the splitting of the spectral lines of atoms due to first order relativistic corrections.
"The gross structure of line spectra is the line spectra predicted by non-relativistic electrons with no spin. For a hydrogenic atom, the gross structure energy levels only depend on the principal quantum number n. However, a more accurate model takes into account relativistic and spin effects, which break the degeneracy of the energy levels and split the spectral lines. The scale of the fine structure splitting relative to the gross structure splitting is on the order of (ZÎ±)2, where Z is the atomic number and Î± is the fine-structure constant, a dimensionless number equal to approximately 7.297Ã—10âˆ’3.
"The fine structure can be separated into three corrective terms: the kinetic energy term, the spin-orbit term, and the Darwinian term." Wikipedia, Fine Structure