Membranous means having the qualities of a membrane, made of membrane, or resembling a thin, flexible sheet of tissue. A membrane is a thin layer that covers, lines, or connects parts of living things.

Your body contains many membranous structures. The lining inside your cheek is membranous: thin, moist, and flexible. Your eardrums are membranous sheets that vibrate when sound waves hit them. Birds have a membranous layer inside their eggs that cushions the growing chick.

Scientists often describe things as membranous when they're talking about delicate biological tissues that act as barriers or dividers. A frog's webbed feet are membranous, with thin tissue stretched between the toes. Some mushrooms have membranous veils that tear as the mushroom grows. Even the wings of bats are membranous: thin skin stretched between long finger bones.

The word emphasizes thinness and flexibility. When a biologist describes something as membranous, she's noting that it's not thick or rigid like bone, but rather pliable like a sheet of wet tissue paper.