Meat, in its broadest definition, is animal tissue used as food. Most often it references to skeletal muscle and associated fat, but it may also refer to non-muscle organs, including lungs, livers, skin, brains, bone marrow and kidneys.
The word meat is also used by the meat packing and butchering industry in a more restrictive sense – the flesh of mammalian species (pigs, cattle, etc.) raised and butchered for human consumption, to the exclusion of seafood, fish, poultry, game, and insects. Eggs are rarely referred to as meat even though they consist of animal tissue.
The word for “meat” is carnis, which does not correspond exactly in meaning to the English word “meat.” In contemporary English, “meat” tends to mean the flesh of any animal, whether it is a mammal, a bird, a fish, or what have you. But as used by the Church, carnis refers only to the flesh of mammals and birds. It does not include the flesh of fish (or, for that matter, of reptiles, amphibians, or insects).
The United States Conference of Catholic Bishops (USCCB), then known as the National Conference of Catholic Bishops, stated in its Pastoral Statement On Penance and Abstinence, November 18, 1966, stated regarding non-Lenten Fridays:
“… the Catholic bishops of the United States, far from downgrading the traditional penitential observance of Friday, and motivated precisely by the desire to give the spirit of penance greater vitality, especially on Fridays, the day that Jesus died, urge our Catholic people henceforth to be guided by the following norms:
1. Friday itself remains a special day of penitential observance throughout the year, a time when those who seek perfection will be mindful of their personal sins and the sins of mankind which they are called upon to help expiate in union with Christ Crucified;
2. Friday should be in each week something of what Lent is in the entire year. For this reason we urge all to prepare for that weekly Easter that comes with each Sunday be freely making of every Friday a day of self-denial and mortification in prayerful remembrance of the passion of Jesus Christ;
3. Among the works of voluntary self-denial and personal penance which we especially commend to our people for the future observance of Friday, even though we hereby terminate the traditional law of abstinence as binding under pain of sin, as the sole prescribed means of observing Friday, we give first place to abstinence from flesh meat. We do so in the hope that the Catholic community will ordinarily continue to abstain from meat by free choice as formerly we did in obedience to Church law.
The bottom line is that Chicken is considered flesh meat and is therefore proscribed.