A hiccup or hiccough (generally pronounced "HICK-cup" (IPA: [ˈhɪ.kəp]) independent of the spelling) is an involuntary spasm of the diaphragm; typically this repeats several times a minute. The sudden rush of air into the lungs causes the glottis to close, creating the "hic" listen (help·info) noise. A bout of hiccups generally resolves by itself, although many home remedies are in circulation to shorten the duration, and medication is occasionally necessary. By extension, the term "hiccup" is also used to describe a small and unrepeated aberration in an otherwise consistent pattern. The medical term is singultus.
While many cases develop spontaneously, hiccups are known to develop often in specific situations, such as eating too quickly, taking a cold drink while eating a hot meal, eating very hot or spicy food, laughing vigorously or coughing, drinking an excess of an alcoholic beverage, or electrolyte imbalance. Hiccups may be caused by pressure to the phrenic nerve by other anatomical structures, or rarely by tumors and certain kidney disease. It is reported that 30% of chemotherapy patients suffer singultus as a side effect to treatment. (American Cancer Society)
It is still unclear to scientists exactly why hiccups occur, particularly because it doesn't seem to give us any specific benefit. Some speculation exists that hiccups are a remnant of a bodily function that has been phased out by evolution, such as the move from aquatic gilled creatures to land animals. Ultrasounds have also shown that unborn fetuses experience hiccups. Some suggested hypotheses include hiccups as a possible muscle exercise for the child's respiratory system prior to birth or as a preventive measure to keep amniotic fluid from entering the lungs. More research is required to ascertain their true nature, origins, and purpose, if any.