The National Bureau of Standards has laboratories that has the task of defining the units in the US
Specifically the pound mass is based on the kiligram (still based on an actual kg mass of platinum/iridium that is in set conditions and inert gas. This is sence the turn of the 20th century. The weight is the actual pound which is just the force exerted by the weight (at aome standard agreed upon conditions of a 1 pound mass. The mass unit was called the slug in the past when the pound (a force unit) was the fundamental unit.
