1. (in anatomy)

1. a. A hollow muscular organ in most vertebrates, also known as the urinary bladder, in which urine is stored before being discharged. In mammals urine is conveyed from the kidneys to the bladder by the ureters and is discharged to the outside through the urethra.

2. b. Any of various other saclike organs in animals for the storage of liquid or gas. See gall bladder; swim bladder.

2. (in botany)

1. a. A modified submerged leaf of certain aquatic insectivorous plants, such as the bladderwort (Utricularia). It forms a hollow with a single opening that is sealed by a valve to trap small aquatic invertebrates after they have been sucked in.

2. b. An air-filled cavity in the thallus of certain seaweeds, such as the bladderwrack (Fucus vesiculosus).