Your abdominal muscles may be hidden under some blubber, but they are there.
I am mainly talking about the rectus abdominis (the source of the six-pack) and the oblique muscles on the side. The oblques are especially used for torso twisting, while the rectus does not exist for doing sit-ups but mainly to work in tandem with our back muscles so that we can stand up - and to bend over when we need to.
Many people exercise these muscles for athleticism or for appearance. Part of exercising "the core," as they say. Is it worth doing? Well, it can't hurt especially as we get older, but almost all calisthenics and heavy weights challenge those core muscles pretty well. Planks are the only core-specific things I do and I do them because I am told to.
It is near-impossible for a woman to show a six-pack, and it seems like a silly pursuit for the average guy. For one thing, it takes a body fat of roughly below 9% to uncover the divisions of the rectus abdominis assembly.
What the ordinary exerciser can achieve with exercise and proper diet is to be "cut" vertically, ie having a clear demarcation between the rectus and the obliques. I think that ought to be a plenty good-enough conditioning indicator for man or woman. It is an indicator of somewhere around 15% body fat and of decent but not extreme fitness.
At 15%, you will still sizzle nicely in the crematorium but you will not start a conflagration. At 25% you are chubby and begin to be a fire hazard.
A pal of mine who works out bragged to me that he doesn't have a lousy six-pack - he has a whole keg.
