As in "anxious", or nervous. In the more severe cases, the symptoms persist due to serotonin imbalances in the brain, rather than through outside stressors. Outside stressors contribute to the initial development of the general anxiety. Severe anxiety occurs when those stressors have effected the brains serotonin levels, causing the symptoms of nervousness to occur seemingly without said outside stressors. The anxious symptoms could then become constant, and tend to only be treatable with medications that effect the levels of serotonin in the Brain. Selective Serotonin Reuptake Inhibitors such as Zoloft, Paxil, and Prozac are commonly prescibed to treat severe chronic anxiety.