Terrier is a type of dog originally bred to hunt vermin. A terrier is a dog of any one of many breeds or landraces of the terrier type, which are typically small, wiry, game, and fearless. There are five different groups of terrier, with each group having different shapes and sizes.

Dog types are broad categories of domestic dogs based on form, function, or style of work, lineage, or appearance. Some may be locally adapted dog types that may have the visual characteristics of a modern purebred dog. In contrast, modern dog breeds strictly adhere to long-established breed standards,[note 1] that began with documented foundation breeding stock sharing a common set of inheritable characteristics, developed by long-established, reputable kennel clubs that recognize the dog as a purebred.