"Jailbreak" is a made up term. As far as I know, it originally referred to accessing arbitrary files on the filesystem (getting out of a chroot jail), but it now refers to a process including gaining access to the bootloader and/or kernel. Either would be an acceptable use of "jailbreak" (though the former might be misleading), but the term has never meant "run unsigned apps". HTH :p
Hmm, my impression was that the process of "jailbreaking" an iPhone was somehow rather related to breaking out of a chroot jail. I don't know where I got that impression though.
