Cant remember name of this process

There is a process that I cant remember the name of. When I learned it there were really great videos on it and everything. Basically am trying to explain this process to a friend and its hard to prove I’m explaining how it works without knowledge of the name. Its when particles pass through a barrier. In this instance something contained in plastic can still inherit the smells of its outside environment. With the name I can look up the video that explains it, which basically just shows moving particles going through a barrier over time.