Definition of "guinea pig" [guin•ea pig]

• Any of various small, short-eared domesticated rodents of the genus Cavia, having variously colored hair and no visible tail. They are widely kept as pets and often used as experimental animals. (noun)
• Informal A person who is used as a subject for experimentation or research. (noun)

American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

• "She was immune to promicin, thanks to playing guinea pig for Kevin Burkhoff a few years back, but Tom was not."
• "Sweetie Pie had been one of the stunted sweet-stuff pigs, and she never quite made it to normal guinea pig size."
• "Even though Sweetie Pie never got much bigger, she was, according to Jancy, the smartest, cutest guinea pig that ever lived."