A packrat was a type of rodent native to the planet Earth.
The term "packrat" was also used as slang to refer to someone who hoarded a large number of items.
In 2259, when questioned by La'an Noonien-Singh as to her large collection of random items, Pelia, a long-lived Lanthanite replied, "Once you have lived through every natural disaster and economic calamity in Human history without becoming a pack rat, then you can judge me." (SNW: "Tomorrow and Tomorrow and Tomorrow")
In 2375, while Ezri Dax was searching the deceased Greta Vanderweg's quarters on Deep Space 9, former Dax symbiont host Joran Dax referred to the lieutenant Commander as having been "quite a packrat". Ezri defended her, noting that Vanderweg had lived on DS9 for a while and considered it home (i.e. she would naturally have accumulated a lot of personal possessions) (DS9: "Field of Fire")
The animal is properly called a "pack rat" though the term is used as "packrat" in the episode script.