Nor[e1, e2, ...] is the logical NOR function. It evaluates its arguments in order, giving False immediately if any of them are True, and True if they are all False.
Nor[e1, e2, ...] is equivalent to Not[Or[e1, e2, ...]]. »
Nor has attribute HoldAll, and explicitly controls the evaluation of its arguments. In Nor[e1, e2, ...] the are evaluated in order, stopping if any of them is found to be True. »
Nor gives symbolic results when necessary, removing initial arguments that are False.