A thin sheet of tissue or layer of cells acting as a boundary, lining, or partition in an organism.
Can also be known as: a selective barrier; it allows things to pass through but stops others.
I believe the answer is: Similarity
According to the gestalt principles, individuals that possess similar objects in term of shapes or colour would be automatically perceived as being in the same group. In public setting such as soccer match, this help the audience distinguish one team with another.