patagium

noun pl. -·gia-ə

1. a fold of skin between the fore and hind limbs of bats, flying squirrels, etc., enabling them to fly or glide through the air
2. a fold of skin between the shoulder and forepart of a bird's wing

Etymology: ModL < L, gold edging of a tunic, border < Gr patageion < patagein, to rustle