Chickens are relatively large birds, active by day. The body is round, the legs are unfeathered in most breeds, and the wings are short. Wild junglefowl can fly; chickens and their flight muscles are too heavy to allow them to fly more than a short distance. Size and coloration vary widely between breeds. Newly-hatched chicks of both modern and heritage varieties weigh the same, about 37g. Modern varieties however grow much faster; by day 35 a Ross 708 broiler may weigh 1.8 kg as against the 1.05 kg of a heritage chicken of the same age.

Adult chickens of both sexes have a fleshy crest on their heads called a comb or cockscomb, and hanging flaps of skin on either side under their beaks called wattles; combs and wattles are more prominent in males. Some breeds have a mutation that causes extra feathering under the face, giving the appearance of a beard.

There are various terms in English that cover the stages in a chicken's lifespan:
Biddy: a chicken, or a newly hatched chicken
Capon: a castrated or neutered male chicken
Chick: a young chicken
Cock: a fertile adult male chicken
Cockerel: a young male chicken
Hen: an adult female chicken
Pullet: a young female chicken less than a year old. In the poultry industry, a pullet is a sexually immature chicken less than 22 weeks of age.
Rooster: a fertile adult male chicken, especially in North America. Originated in the 18th century, possibly as a euphemism to avoid the sexual connotation of the word cock.
Yardbird: a chicken (southern United States, dialectal)

Chickens are descended primarily from the red junglefowl (Gallus gallus) and are scientifically classified as the same species. Domesticated chickens freely interbreed with populations of red junglefowl. The domestic chicken has subsequently hybridised with grey junglefowl, Sri Lankan junglefowl and green junglefowl; a gene for yellow skin, for instance, was incorporated into domestic birds from the grey junglefowl (G. sonneratii). It is estimated that chickens share between 71 and 79% of their genome with red junglefowl.

Exactly when and where the chicken was domesticated remains controversial. Genomic studies estimate that the chicken was domesticated 8,000 years ago in Southeast Asia and spread to China and India 2,000 to 3,000 years later. Archaeological evidence supports domestic chickens in Southeast Asia well before 6000 BCE, China by 6000 BCE and India by 2000 BCE. A landmark 2020 Nature study that fully sequenced 863 chickens across the world suggests that all domestic chickens originate from a single domestication event of red junglefowl whose present-day distribution is predominantly in southwestern China, northern Thailand and Myanmar. These domesticated chickens spread across Southeast and South Asia where they interbred with local wild species of junglefowl, forming genetically and geographically distinct groups. Analysis of the most popular commercial breed shows that the White Leghorn breed possesses a mosaic of divergent ancestries inherited from subspecies of red junglefowl.

Typical chicken jointed cuts:

Cuts of Chicken:
Breast
Leg (divided into drumsticks and thighs)
Wings (divided into tips and drumettes)
Offal — typically hearts, necks, crops (gizzards), feet, unlaid eggs and combs