Vitreous refers to a material in a glassy state. In such a state, the constituent atoms do not exhibit the long-range order that is characteristic of crystals. However, they still exhibit short-range order -- the separation of atoms and/or the lengths of covalent bonds are very close to their typical equilibrium distances.
Under certain conditions, vitreous materials may begin to rearrange themselves into crystals. This process is referred to as devitrification.
See the entry on glass for more information.
In Ophthalmology, the inner part of the eye which is made up of a clear jelly-like fluid is known as vitreous humour.