The bitter melon [Momordica charantia] is an unusually shaped vegetable with a acquired, bitter taste. Its white fibrous seed core is usually cut in half and seeded, then the interior membrane is discarded, leaving a thin ring of flesh. It is used in soups and stir-fries, or can be steamed and braised.
In other countries the bitter melon is referred to as a bitter gourd. There are many varieties of this vegetable which is similar to a summer squash or a cucumber. They can be lighter or darker in color and some have a decidedly bumpy exterior. Fruits which have turned yellow are usually considered overripe and turn a little sweet and are not used.
Depending on where you live this vegetable is typically sold in grocery stores that cater to the Asian community or possibly a well-stocked main stream grocer in an area with a large Asian population. Other specialty stores such as Thai, or Indian markets will probably carry them as well.