The definition of a hiccup is an involuntary spasm of the diaphragm along with a rapid closure of the glottis that causes you to make an audible sound.
When you drink too much soda and your diaphragm spasms and you make a gasp-like sound, this is an example of a hiccup.
See hiccup in Webster's New World College Dictionary
Origin: altered < Early ModE hikop, hickock, hicket, of echoic orig. (as also in MDu huckup): var. sp. from assoc. with cough
See hiccup in American Heritage Dictionary 4
Learn more about hiccup