Nutmeg is the seed inside the fruit of the nutmeg tree. We sell it whole and we would discourage you from buying it any other way.
The reason is simple. Nutmeg's aroma sits in volatile oils that begin escaping the moment the seed is broken. Ground nutmeg is noticeably flat within a few weeks and close to pointless within a few months, which is why the jar in most kitchens tastes of very little.
Each seed is dried slowly until it rattles inside its shell, then cracked out and sorted by size. A single nutmeg will last most households a year of regular grating.
The same fruit produces mace, the lacy red covering around the seed, which we also sell. They taste related but they are not interchangeable.