  Crystals and quasicrystals can be characterized by an order that is a purely
geometric property of an instantaneous configuration, independent of particle
dynamics or interactions. Glasses, on the other hand, are ostensibly amorphous
arrangements of particles. A natural and long-standing question has been
whether they too have, albeit in a hidden way, some form of geometric order.
  Here we define a coherence length that applies to systems which are typically
characterized as amorphous, as well as to those that are conventionally
ordered. We argue that the divergence of such a length is consistent with
current theories of the `ideal' glass transition.
