The condition of dry snow after it has been sitting around for a while.
True packed powder, if you're not familiar with snow, is similar in consistancy to that of sand, but much lighter. That is, it's fairly flat, and doesn't
blow around loosely.
The term is much overused by ski resorts, because it sounds better to say:
"Conditions today are fantastic. An 80 cm base with a 10 cm cover of packed powder...."
sounds much better than:
"Conditions today are fantastic. An 80cm base with a 10cm cover of ice...."
This given, packed powder describes pretty much any kind of snow except
fresh powder.