The American Heritage® Dictionary of the English Language, 4th Edition
n. A small, brightly colored live-bearing freshwater fish (Poecilia reticulata or Lebistes reticulatus), native to northern South America and adjacent islands of the West Indies and popular in home aquariums.
n. A tiny freshwater fish, Poecilia reticulata, popular in home aquariums, that usually has a plain body and black or dark blue tail for the females and a more colorful tail for the males.
n. By extension, any tiny fish.
the GNU version of the Collaborative International Dictionary of English
n. A small freshwater fish of South America and West Indies (Lebistes reticulatus or Poecilia reticulata, originaly called Gerardinus guppyi), often kept in aquariums; also called rainbow fish.
WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
n. small freshwater fish of South America and the West Indies; often kept in aquariums
Word Usage
"Jeffcoat said he booked a reservation on the "guppy" - a nickname that refers to the plane's stubby, round profile - when he heard of the retirement."