...AND HIS TRUSTY MOUNT, FIRESTORM THE WONDER HORSE

Oxidative stress is caused by an imbalance between the production of reactive oxygen and a biological system's ability to readily detoxify the reactive intermediates or easily repair the resulting damage. All forms of life maintain a reducing environment within their cells. This cellular redox environment is preserved by enzymes that maintain the reduced state through a constant input of metabolic energy. Disturbances in this normal redox state can cause toxic effects through the production of peroxides and free radicals that damage all components of the cell, including proteins, lipids, and DNA.

To put it in terms your General Chem prof would have understood, oxidative stress is a large increase (ie, more negative) in cellular reduction potential, which is the tendency of a chemical species to sop up electrons and become reduced. You could also call it a decrease in the reducing capacity of cellular redox couples.