The condition of having a body weight high enough to pose a severe risk to health. This is informally measured as having a body weight which is more than twice the optimum. It is indicated more precisely by a 'body mass index' of 40 or over (BMI = the weight in kilograms divided by the square of height in metres).
It could be seen as a sign of the times in the English-speaking world that, prior to the latter half of the 20th century, we got by perfectly well without feeling the need to give a name to this condition. In 1969, the American Journal of Surgery defined the term:
"We identify morbid obesity as existing in any person whose weight has reached a level two or three times his ideal weight and who has maintained this level of obesity for five years or more."
It is interesting to see the use of the word morbid in its original medical sense. Like chronic, morbid is more often used loosely and both words are in danger of loosing their original meaning. Many people use chronic to mean severe rather than its correct meaning, which is 'pertaining to time' - usually long-lasting. So, 'chronic fatigue' is often thought of as fatigue that is severe and debilitating rather than what it really is - fatigue which is persistent and ongoing. It may be severe also of course. Likewise, 'morbid' is most often used to mean gloomy and depressing and this has become accepted as correct usage. The medical description 'affected with disease' was the word's proper meaning. So, 'morbid obesity' isn't broody or gloomy but simply obesity that is severe enough to be considered a threat to health.
As an aside, 'disease' is itself interesting as an example of a word that, when spelled with its hyphen, is much easier to understand; for example, from George Chapman's Odyssey, 1615:
"Doth sleep thus seize Thy powers, affected with so much dis-ease?"