Croup, kroop, is an inflammation of the air passages of the throat and trachea (windpipe). During an attack of croup, the mucous membranes in these areas become inflamed and swollen, restricting the flow of air. A victim of croup feels hoarse, breathes with great difficulty, and wheezes when inhaling. The patient also develops a hollow, barking cough. Croup occurs most often among children who are 6 months to 3 years old. Their air passages are smaller and more easily blocked.