It is the upper part of a pig’s rear thigh.
It is fattier than ham from the lower part of the pigs leg (the “shank” half), but easier to carve, and often more expensive.
If it’s labelled “butt portion”, the centre ham slice has been removed for sale separately labelled as a “ham steak” or “centre cut ham slice.”
If it’s labelled “butt half”, the centre slice is still there.
Cook’s Illustrated magazine recommends the shank end of the leg, but many butchers actually prefer this cutt, the butt end.