Googled "his midriff" - 489,000 Googled "his abdomen" - 3 million Googled "her midriff" - 3.45 million Googled "her abdomen" - 1.6 million . Not sure whether you wanted the gender bias included. :)

'Midriff' is the female equivalent to the male word 'six-pack.' When a girl wears clothing that exposes her abdomen, it's called "a midriff." If the girl is fat and there is a mushroom looking bit of fat hanging over her pants or jeans, it is called a muffin top, and it is also called a muffin top when you can see it under her shirt. When a boy 'goes shirtless,' (goes out in public without a shirt on), his 'six-pack' is exposed. It is only a 'six-pack' if his abdominal muscles (or abs) are clearly visible and well-defined, if he has a round, fat abdomen, (or gut), it is called "a beer belly" NOT a 'six-pack!' Whew!

Well, strictly, isn't the midriff used to describe what you see, and the abdomen the body volume roughly under the midriff? So, is the pain on the surface, or deep inside ...?

In this case, I suppose doctors would use "abdomen" if they have a brain.

Abdomen. It's more of a biological term, describing the lower part of the trunk, below the ribs. By contrast, midriff is a term generally used when you can see a person's waist or stomach. E.g. "that top shows off your midriff"