Anything related to the behavior and properties of electrons, i.e. the elementary subatomic particles accounting for electrical conduction in most solid materials, especially metals.
Electron (e- or -1e0) is a negatively-charged (-1), lightweight-type subatomic-particle with possible spin state +1/2 or -1/2.
Electrons form a diffused, negatively-charge quantum-cloud (differentiated into shells, subshells and orbitals), around the nucleus (that is a heavyweight, compact lump of particles protons (+1 charged) and neutron (Zero-charged)). The electronic configuration determines most physical properties and all chemical properties of a substance.
A group of electrons, loosely bound to the atom, called "Free electrons", that exist in "conduction band", cause good electrical and thermal conductivity in metals, metal-alloys and such loose-electrons conduct electricity in semiconductors.
Isolated electrons found in the electron-beams (cathode-ray) at CRT(Cathode-Ray-Tube) , and the "space-charge" in vacuum tubes (valves). The word "Electronics" is direct derivative of the term electron. The physics of the particle electron, is key to both solid-state electronics and Vacuum, low-pressure-gas and plasma electronics.
See also Wikipedia on electrons.