From AMS Glossary
Pure liquid water is optically homogeneous over the visible spectrum because one cubic wavelength of water contains many molecules, whereas a cloud of water droplets is not optically homogeneous. Optical homogeneity is more general than transparency, usually restricted to visible wavelengths. A body is said to be transparent if it transmits images. Optical homogeneity is necessary for transparency but not sufficient. A sufficiently thick sample of an absorbing optically homogeneous material would be described as opaque rather than transparent.