The energy gap between a semiconductor’s valence and conduction bands is called the band gap.
In a semiconductor, electrons normally occupy the valence band, while the conduction band contains higher-energy states in which electrons can move through the crystal and carry current. The band gap is the energy difference between the top of the valence band and the bottom of the conduction band. It is commonly measured in electronvolts, or eV.
An electron must receive at least enough energy to cross this gap. Heat, light, or another source can provide that energy, leaving behind a positively charged vacancy called a hole. The number of available electrons and holes strongly affects a semiconductor’s conductivity, which is why band-gap size is central to electronics and optoelectronics.
Band gaps can be direct or indirect. In a direct-gap material, an electron can transition while conserving crystal momentum through photon interaction alone; indirect transitions also involve a lattice vibration, or phonon. “Forbidden gap” is a related descriptive phrase, but band gap is the standard canonical answer.