n.1 a any of various membranous sacs in some animals, containing urine (urinary bladder), bile (gall-bladder), or air (swim-bladder). b this or part of it or a similar object prepared for various uses.
2 an inflated pericarp or vesicle in various plants.
3 anything inflated and hollow.
Etymology: OE bl{aelig}dre f. Gmc