$e_g$:The $e_g$ term refers to the other set of molecular orbitals formed from the interaction between the central metal atom's d-orbitals and the ligand orbitals in an octahedral complex.
Ligand Field Theory:Ligand Field Theory describes the splitting of the metal atom's d-orbitals into $t_2$ and $e_g$ sets when the atom is surrounded by ligands in an octahedral arrangement.
Crystal Field Theory:Crystal Field Theory is a model used to explain the bonding, magnetic properties, and electronic transitions in coordination compounds, which involves the splitting of the metal atom's d-orbitals into $t_2$ and $e_g$ sets.