A packrat, also called a woodrat, can be any of the species

Species

In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

 in the rodent genus Neotoma. Packrats have a rat-like appearance, with long tails, large ears and large black eyes. Compared to deer mice, harvest mice and grasshopper mice, packrats are noticeably larger and are usually somewhat larger than cotton rats.

A common accepted spelling of "packrat" includes "pack rat".