From the Terminal, type:
defaults write com.apple.Terminal FocusFollowsMouse -string YES
Quit and restart the Terminal and enjoy!
This change is only for your account – and is not machine wide – so if you’re using someone else’s machine with a networked home directory, this preference should follow you.
I know all you Linux/X11 users who are going to love this one! If you don’t like it, though, just re-do the command with NO at the end instead of YES.
If you want to use the same feature in the X11 interface, simply use the following line in the X11 command window:
defaults write com.apple.x11 wm_ffm true
To turn it off here, replace the true with false