noun

1. a bag consisting of or lined with membranous tissue in the body of many animals, capable of inflation to receive and contain liquids or gases; esp. the urinary bladder in the pelvic cavity, which holds urine flowing from the kidneys
2. a thing resembling such a bag, as the inflatable rubber bag inside the leather cover of a football
3. 1. an inflated covering of certain fruits
   2. an air sac, as in some water plants

Etymology: ME bladre < OE blæddre < IE *bhlē-: see blast