In Commonwealth countries, poussin is a butcher's term for a young chicken

Chicken

The chicken is a domesticated fowl, a subspecies of the Red Junglefowl. As one of the most common and widespread domestic animals, and with a population of more than 24 billion in 2003, there are more chickens in the world than any other species of bird...

, less than 28 days old at slaughter and usually weighing 400-450 grammes but not above 750g. It is sometimes also called spring chicken, although the term spring chicken usually refers to chickens weighing 750-850g.

In the United States of America, poussin is an alternative name for a small-sized cross-breed chicken called Rock Cornish game hen

Cornish game hen

In the USA, a Cornish game hen, also sometimes called a Cornish hen, poussin, Rock Cornish hen, or simply Rock Cornish, is a hybrid chicken sold whole. Despite the name, it is not a game bird, but actually a type of domestic chicken. Though the bird is called a "hen", it can be either male or female...

, developed in the late 1950s, which is twice as old and twice as large as the typical British poussin.

Sources

• The British Department for Environment, Food and Rural Affair' definition of poussin and coquelet
• The British Assured Food Standards organisation's definition of poussin.