The Jack Russell terrier is a small terrier that has its origins in fox hunting. It is principally white-bodied smooth, rough or broken-coated which is commonly confused with the Parson Russell terrier (the American Kennel Club (AKC) and affiliate variant) and the Russell terrier (a shorter legged, stockier variety, whose name within the Fédération Cynologique Internationale is "Jack Russell terrier"), with the term "Jack Russell" commonly misapplied to other small white terriers. The Jack Russell is a broad type, with a size range of 10–15 inches (25–38 cm), the Parson Russell is limited only to a middle range with a standard size of 12–14 inches (30–36 cm), while the Russell terrier is smaller at 8–12 inches (20–30 cm), however each breed has different physical proportions according to the standards of their breed clubs.
Originating in the early 19th century from dogs bred and used by Reverend John Russell, it has similar origins to the modern Fox terrier. The Jack Russell is an energetic breed which relies on a high level of exercise and stimulation, and is relatively free from serious health complaints. It has gone through several changes over the years, through different use and breed standards set by kennel clubs. Recognition for the breed by kennel clubs has been opposed by the breed's parent societies – which resulted in the breeding and recognition of the Parson Russell terrier. Jack Russells have appeared many times in film, television and print with several historical dogs of note.