Anxiety is an unpleasant, emotional state of high energy that involves a complex combination of emotions that include fear, apprehension, and worry. It is often accompanied by physical sensations such as heart palpitations, nausea, chest pain, shortness of breath, or tension headache.

Anxiety is often described as having cognitive, somatic, emotional, and behavioral components. The cognitive component entails expectation of a diffuse and uncertain danger. Somatically the body prepares the organism to deal with threat: blood pressure and heart rate are increased, sweating is increased, blood flow to the major muscle groups is increased, and immune and digestive system functions are inhibited. Externally, somatic signs of anxiety may include pale skin, sweating, trembling, and papillary dilation. Emotionally, anxiety causes a sense of dread or panic and physically causes nausea, and chills. Behaviorally, both voluntary and involuntary behaviors may arise directed at escaping or avoiding the source of anxiety. These behaviors are frequent and often maladaptive, being most extreme in anxiety disorders. However, anxiety is not always pathological or maladaptive: it is a common emotion along with fear, anger, sadness, and happiness, and it has a very important function in relation to survival.

Neural circuitry involving the amygdala and hippocampus is thought to underlie anxiety. When confronted with unpleasant and potentially harmful stimuli such as foul odors or tastes, PET-scans show increased blood flow in the amygdala. In these studies, the participants also reported moderate anxiety. This might indicate that anxiety is a protective mechanism designed to prevent the organism from engaging in potentially harmful behaviors.