/ek'steuhr nal"i tee/ , n. , pl. externalities .
1. the state or quality of being external.
2. something external; an outward feature.
3. excessive attention to externals.
4. an external effect, often unforeseen or unintended, accompanying a process or activity: to eliminate externalities such as air pollution through government regulation.
[ 1665-75; EXTERNAL + -ITY ]