A.K.A. Sitz bones, sitting bones, buttock bones part of the pelvis; the two bony protuberances that are the lowest part of your pelvis; ischial tuberosities.
If you have taken a yoga class, your teacher probably asked you at some point to sit on your sit bones. I spent years wondering exactly what she (the teacher was usually a she) was saying (sit? sitz? some word in Sanskrit that I don't know?) and doing my best. It's hard to do something properly when you don't know the parts you are supposed to be using.
Sit bones are hard to feel, since the large muscle and fat of our gluteus maximus provides plenty of padding.
I have a new yoga teacher at the Y, who made it perfectly clear in one easy lesson: she asked us to sit on the floor, legs extended in front of us, and 'walk' forward by rocking our hips side to side. When you do that, you clearly feel your sit bones against the floor.
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Another time you might be aware of your sit bones is when bicycling (or more likely, the day after a long bike ride). If you don't have a bicycle seat that fits you well, most of your weight rests on the sit bones, which can be painful after a while. There are videos and websites devoted to helping you measure your sit bones, so that you can get a proper fit.
DonJaime says re sit bones: "I think, if the bike saddle fits properly you are sitting on the bones as well. Just better." He's right. It turns out that positioning on the bike--how far forward you lean, from a triathlete's posture to fully upright--is a factor in the kind of seat you need, as is your gender--bone structure and spacing is different for men and women. Proper alignment is everything.
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Sitz Bones & the Art of Sitting
Sit bone measurement
Saddle Ergonomics Explained