The electron affinity (E) of an atom or molecule is defined as the amount of energy released when an electron attaches to a neutral atom or molecule in the gaseous state to form an anion.
::X(g) + e →  X(g) + energy
This differs by sign from the energy change of electron capture ionization. The electron affinity is positive when energy is released on electron capture.
In solid state physics, the electron affinity for a surface is defined somewhat differently (see below).