amount of energy released when an electron is attached to a neutral atom or molecule in the gaseous state to form a negative ion
Electron affinity is a phrase used in chemistry. When an atom that is neutral gets an electron, the energy that is changed in the atom is the electron affinity. Across a period, the electron affinity increases. 
- Davis, Raymond E., Regina Frey, and Mickey Sarquis. Modern Chemistry. Holt, Rhinehart, and Wintston. Print.