- a short-eared, tailless rodent of the genus Cavia, usually white, black, and tawny, commonly regarded as the domesticated form of one of the South American wild species of cavy: often used in scientific experiments or kept as a pet.
- the subject of any sort of experiment: I've volunteered as a guinea pig in several clinical trials.
- a domesticated cavy, probably descended from Cavia porcellus, commonly kept as a pet and used in scientific experiments
- a person or thing used for experimentation
