Look up have butterflies in one's stomach in Wiktionary, the free dictionary.

Butterflies in the stomach is the physical sensation in humans of a "fluttery" feeling in the stomach, caused by a reduction of blood flow to the organ. This is as a result of the release of adrenaline and Norepinephrine in the fight-or-flight response, which causes increased heart rate and blood pressure, consequently sending more blood to the muscles.^[1]

Butterflies in the stomach are usually linked in culture and language to the sentiment of love and romantic passion to a desired other. One may feel butterflies in the stomach prior to meeting or confronting a love interest due to high levels of emotion and anxiety, as adrenaline and serotonin may be released when a love interest is concerned.

It can also be a symptom of social anxiety disorder. The symptom of this phenomenon is usually experienced prior to attempting to partake in something critical.^[2]