Butterflies, in the context of nervousness, refer to the fluttery sensation often felt in the stomach during anxious moments. This feeling is caused by the body's response to stress, where adrenaline is released, leading to changes in digestion and blood flow. This reaction is part of the fight-or-flight response, which prepares the body to face or escape perceived threats. While commonly experienced before events like public speaking or exams, these butterflies are a normal physiological response to anxiety and can vary in intensity from person to person.