man xterm

OR if you prefer

man aterm

It shows all options available. You can use many options not just transparency but also shade (in case you use a light background for example) or setting the scrollback buffer. I prefer Eterm and use a script that launches it like so:
Code:
Eterm -O --shade 40 --resize-gravity -L 5000 --home-on-input -f rgb:00/ff/00 --color10 rgb:ff/ff/ff --color12 rgb:99/99/ff
Many options (but certainly not all) are the same no matter what terminal you like.