:what is the meaning of "ad hoc query"?

hi,all

	i am reading Oracle Discoverer handbook and see a word "ad hoc query". but i don't know the exact meaning of this, nor can i find the meaning in dictionary. i am a non-native english speaker, and hope the list can explain it to me.
	thanks a lot.
	if any chinese here, would you kind to give a chinese translation of this?
	regards.
            zhuchao
            zhuchaopku_at_263.net