Bleb: A bladder-like structure more than 5 mm in diameter with thin walls that may be full of fluid. Also called a bulla. SLIDESHOW Heart Disease: Causes of a Heart Attack See Slideshow From Healthy Resources 10 Rheumatoid Arthritis SymptomsWhat Is Exocrine Pancreatic Insufficiency?Truth About Vitamin B12 Featured Centers Good and Bad Foods for PsoriasisVideo: Getting Personal on Life With MS Health Solutions From Our Sponsors Shot-Free MS Treatment Your Child and COVID-19