A spaniel is a type of gun dog. Spaniels were especially bred to flush game out of denser brush. By the late 17th century, spaniels had been specialized into water and land breeds. The extinct English Water Spaniel was used to retrieve water fowl shot down with arrows. Land spaniels were setting spaniels—those that crept forward and pointed their game, allowing hunters to ensnare them with nets, and springing spaniels — those that sprang pheasants and partridges (for hunting with falcon) and also rabbits and smaller mammals such as rats and mice (for hunting with greyhounds). During the 17th century, the role of the spaniel dramatically changed as Englishmen began hunting with flintlocks for wing shooting. Charles Goodall and Julia Gasow (1984) write that spaniels were "transformed from untrained, wild beaters, to smooth, polished gun dogs."

The word "spaniel" would seem to be derived from the medieval French espaigneul—"Spanish"—to modern French, espagnol.

The Oxford English Dictionary defines Spaniel as "a breed of dog with a long silky coat and drooping ears".

Not much has changed about spaniels in general over the years, as can be seen in this 1921 entry in Collier's New Encyclopedia:

Their distinguishing characteristics are a rather broad muzzle, remarkably long and full ears, hair plentiful and beautifully waved, particularly that of the ears, tail, and hinder parts of the thighs and legs. The prevailing color is liver and white, sometimes red and white or black and white, and sometimes deep brown, or black on the face and breast, with a tan spot over each eye. The English spaniel is a superior and very pure breed. The King Charles is a small variety of the spaniel used as a lapdog. The water spaniels, large and small, differ from the common spaniel only in the roughness of their coats, and in uniting the aquatic propensities of the Newfoundland dog with the fine hunting qualities of their own race. Spaniels possess a great share of intelligence, affection, and obedience, which qualities, combined with much beauty, make them highly prized as companions.

The origin of the word spaniel is described by the Oxford English Dictionary as coming from the Old French word espaigneul which meant "Spanish (dog)"; this in turn originated from the Latin Hispaniolus which simply means "Spanish".

In Edward, 2nd Duke of York's work The Master of Game, which was mostly a 15th-century translation of an earlier work by Gaston III of Foix-Béarn entitled Livre de chasse, spaniels are described as being from Spain as much as all Greyhounds are from England or Scotland. Sixteenth-century English physician John Caius wrote that the spaniels of the time were mostly white, marked with spots that are commonly red. He described a new variety to have come out of France, which were speckled all over with white and black, "which mingled colours incline to a marble blewe".

In the appendices added to the 1909 re-print of Caius' work, the editors suggested that the type of dogs may have been brought into the British Isles as early as 900 BC by a branch of the Celts moving from Spain into Cornwall and on into Wales, England and Ireland. Theories on the origin of the Welsh Springer Spaniel support this theory, as it is believed that the breed specifically is a direct descendant of the "Agassian hunting dog" described in the hunting poem Cynegetica attributed to Oppian of Apamea, which belonged to the Celtic tribes of Roman Britain: