I can't quite see why you'd want this, unless you only
ever worked in terminal, but mine is not to reason why 🙂
You have three options that I can think of, depending on what your goals are.
First, if you want to keep the terminal
active, then a background AppleScript that periodically activates Terminal.app would do that. However, this could get annoying if you ever want to run another app (say, the Finder?) and you keep getting dropped back into Terminal.
Secondly, and what I'm guessing is closer to what you want, is Focus-follows-mouse - a mode where simply pointing the mouse at a terminal window gives that window focus (e.g. all typing is directed there) without bringing that window frontmost.
There are a couple of levels this works at. If you're just focussed (no pun intended) on Terminal.app then you can run:
<pre class=command>defaults write com.apple.Terminal FocusFollowsMouse 1</pre>
when Terminal isn't active, and next time you'll have mouse focussing.
The third option which might get you what you want is
Visor - a widget that brings up a shell window overlaying your current display when you press a hotkey. This is the only method I'm aware of to have a shell truly on top of any other window, but it's most useful for intermittent use (e.g. you just need to run a quick command or two), not for continuous use, but it might fit your needs.
