membrane

Short definition from Wiktionary

Noun

1. 1
   A flexible enclosing or separating tissue forming a plane or film and separating two environments: A mechanical, thin, flat flexible part that can deform or vibrate when excited by an external force.
2. 2
   A flexible enclosing or separating tissue forming a plane or film and separating two environments: A flexible or semiflexible covering or waterproofing whose primary function is to exclude water.
3. 3
   A flexible enclosing or separating tissue forming a plane or film and separating two environments: A microscopic double layer of lipids and proteins forming the boundary of cells or organelles.
4. 4
   A piece of parchment forming part of a roll.