A hiccup is an unintentional movement of the diaphragm, the muscle at the base of the lungs. It's followed by quick closing of the vocal cords, which produces the "hic" sound you make. There are a large number of causes, including large meals, alcohol, or hot and spicy foods. Hiccups may also start and stop for no obvious reason.
There is no sure way to stop hiccups. You can try
Hiccups aren't usually serious. Contact your health care provider if they last for more than a few days.
References and abstracts from MEDLINE/PubMed (National Library of Medicine)