A mastiff is a large and powerful type of dog.^[1][2] Mastiffs are among the largest dogs, and typically have a short coat, a long low-set tail and large feet; the skull is large and bulky, the muzzle broad and short (brachycephalic), and the ears drooping and pendant-shaped.^[1][2] European and Asian records dating back 3,000 years show dogs of the mastiff type.^[3] Mastiffs have historically been guard dogs, protecting homes and property, although throughout history they have been used as hunting dogs, war dogs and for blood sports, fighting each other and such animals as bulls, bears and lions.^[2][3]

Historical and archaeological evidence suggests that mastiffs have long been distinct in both form and function from the similarly large livestock guardian dogs from which they were likely developed;^[4] they also form separate genetic populations.^[5] The Fédération Cynologique Internationale and some kennel clubs group the two types together as molossoid dogs; some modern livestock guardian breeds such as the Pyrenean Mastiff, the Spanish Mastiff and the Tibetan Mastiff have the word "mastiff" in their name.^[6]

Many older English sources refer to mastiffs as bandogs or bandogges, although technically the term "bandog" meant a dog that was tethered by a chain (or "bande") that would be released at night, the terms mastiff and bandog were used interchangeably.^[7][8] In the 20th century, the term bandog has been revived to describe some lines of large mastiff-type guard dogs.^[7]