I THINK that SOME terminals do not have to be linked to a network; however others are totally dependent on the network.
EDIT: OK, this from my father, who should know terminals: a "terminal" is reliant on another host computer to fully function, so it must be connected to the network to properly function. Sometimes, these are called workstations. But, in the modern usage of the word "workstation", the machine could be self-contained, but is intended to be used on a network.
(The first part rings a bell now from when we used to own a WYSE Terminal...)