1. Repeated involuntary spasms of the diaphragm followed by sudden closures of the glottis, which checks the inflow of air and produces the characteristic sound.
2. An extraordinary type of breathing movement that involves a sudden intake of air, or inspiration, due to a sudden involuntary contraction of the diaphragm, accompanied by the closure of the glottis (opening between the vocal cords) in the larynx (vocal cords area).

The closure of the glottis then halts the incoming air and the column of air strikes the closed glottis to produce the characteristic sounds of hiccups.

Transient episodes are common; while "persistent hiccups" (up to two days) and "intractable hiccups" (up to one month) are uncommon and quite distressing for the person who is experiencing the condition.

Singultus is "medicalese" for "hiccup" and comes from a Latin element that means "a gasp" or "a sob"; especially, those which are repeated.