noun
1.
[probably so called from the fact that Guinea represented the name of a distant country]
a. : a small stout-bodied short-eared nearly tailless domesticated hystricomorph rodent (genus Cavia ) often kept as a pet and widely used in biological research, occurring in many combinations of black, white, and tawny red, having short or long hair, and being commonly considered a distinct species ( C. cobaya ) although probably a domesticated variety of some So. American species (as C. porcellanus or C. cutleri ) — called also cavy ; see aperea
b. : an animal of Cavia or of related genera — often used with a qualifying term
mountain guinea pigs of the genus Microcavia
2. Britain
a.
[so called from the payment of guineas as fee to the vessel's captain]
: midshipman
b. : a person receiving a guinea as a fee — often used of a doctor or clergyman substituting for another
3.
[so called from the wide use made of guinea pigs in experimentation and research]
: a subject of experimentation or testing designed to yield data for drawing scientific conclusions or large-scale calculations