Featherless means without feathers. Birds are usually covered in feathers, but when they hatch from their eggs, many baby birds are completely featherless, looking pink and vulnerable. Their feathers grow in over the following days and weeks.

The word can describe birds that have lost their feathers through molting (a natural process where old feathers fall out and new ones grow in) or through injury or illness. A featherless chicken might look strange and need extra care to stay warm, since feathers work like a built-in coat.

Scientists and philosophers sometimes use “featherless biped” as a humorous way to define humans. A biped is any creature that walks on two legs, and since humans walk on two legs but have no feathers, technically we fit that description. The ancient Greek philosopher Plato once defined humans as “featherless bipeds,” and his rival Diogenes responded by plucking a chicken and declaring, “Behold! I've brought you a man!” This joke from 2,400 years ago shows how tricky it can be to define things precisely.