The portion of the front leg above the hocks and below the Pork Butt Roast (Boston Butt). See also Picnic Ham for the smoked and cured version. This is a large cut which is often roasted intact, skin-on and bone-in, and a very fine roast it is. It's a fairly lean cut with plenty of meat and big bones through the center, and any chef will tell you there's lots of flavor near the bone.
If you can't handle skin and bones see Pork Butt Roast. The skin and bones are critical flavor elements and you aren't going to get anything near the same without them. There is also a Boneless Pork Picnic Shoulder listed, but it's a disorderly cut and not common around here,
The Picnic Shoulder is also a very interesting piece for parting out, particularly if you have recipes requiring skin (the whole underside of the unit in the photo is covered with skin). An 8 pound shoulder will yield 4-1/2 pounds of clear meat without fat, 10 ounces of skin, only 1/3 cup of fat rendered from the cracklings and a good 2 quarts of soup stock, particularly good if you splinter the bones.
Keep in mind this is working muscle so it has more flavor but needs more cooking time to bring it tender than a pork loin or rib roast would.