Vitreous humour: The fluid between the retina and the lens. It is an absolutely transparent liquid. The liquid is made up of99 per cent water as well as a lower amount of collagen, salt, and sugar. It is a clear gel-like substance that occupies the space behind the lens and in front of the retina at the back of the eye. As people age, vitreous thins. This can result in a condition called posterior vitreous detachment, where the vitreous separates from the retina.