epiphenomenon

Short definition from Wiktionary

Noun

1. 1
   An activity, process, or state that is the result of another; a by-product, a consequence.
2. 2
   A mental process or state that is an incidental by-product of physiological events in the brain or nervous system.
3. 3
   A symptom that develops during the course of a disease that is not connected to the disease.