Transparent material allows to see through it to a certain degree so that material in its background is visible. Transparency is output format dependent and requires special driver code for each format.
I common way to have transparent material is the
opacity option of TikZ/PGF (see tikz-pgf). Here
opacity=1 stands for full opaque content (no transparency at all),
opacity=0 for full transparency (material is actually invisible) and
semitransparent for 50% transparency. Any value between 0.0 and 1.0 can be used.