Shield is a positive status effect in Final Fantasy VII given by the spell of the same name. When under its effect, the target voids all normal attacks and absorbs all elemental damage, though they can still be damaged by items (not when used in W-Item) and non-elemental special attacks. Shield status can be removed by DeBarrier and DeSpell.
Shield is a status effect that while active, nullifies most physical or magic attacks are nullified and when it does the buff fades. Shield does not expire with the passage of time. If Mirror Image and Shield are both active and a valid physical attack is nullified, Mirror Image will fade first. Similar thing happens when Magic Barrier and Shield are both active and a valid magic attack is nullified, Magic Barrier will fade first.