We English speakers use jökulhlaup as if it were synonymous with 'glacial lake outburst flood' -- a flood caused when the lake of water formed from the melting glacier breaks free and spills over the surrounding countryside. The word comes to us from Icelandic, in which jökulhlaup refers specifically to an outburst flood of meltwater, either glacial or trapped in an ice sheet, which is caused by a volcanic eruption under the ice. I will be using jökulhlaup in the more general sense in this writeup.
Glacial outburst floods, under any name, can be caused by volcanoes, icefalls and avalanches, an earthquake or ice quake, or a simple buildup of water pressure. They can be really, really big; one that took place in the Vatnajökull Ice Sheet of Iceland (November 5, 1996) released from three to four cubic kilometers of water, over the period of two days. We have them in America too: in Wyoming, the Grasshopper Glacier let loose 3,217,600m3; the Tulsequah Glacier in Alaska causes floods every few years, flooding a nearby airstrip and endangering local cabins.
Areas in central Asia, the Andes of South America, and the Alps in Europe are also at risk for jökulhlaup flooding. At the moment, the United Nations has marked the Trakarding Glacier, in the Rolwaling Valley of Nepal, as the greatest potential jökulhlaup disaster. The glacier's lake, Tsho Rolpa glacier lake, is growing yearly, with a current volume of about 100 million m3 of ice cold water. The only thing keeping all that water up there is a moraine dam (a mound of accumulated earth an stone) about 150 meters thick.
Jökulhlaup is pronounced YO-kul-hloip, and comes from the Icelandic words jökull meaning "glacier," and hlaup meaning "flood burst" (SharQ points out that hlaup also means 'leap' and 'run', giving the word a more explosive flavor than the English 'flood burst').