Shield (シールド, Shīrudo?) is a recurring status effect in the Final Fantasy series. It generally allows the affected character to avoid attacks or physical damage, but the specific vary in every game it appears in.
Appearances
Final Fantasy VII
Shield is a positive status effect 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.
Aeris can also bestow the status Peerless that makes one immune to all damage.
Final Fantasy VII Remake
The 
Shield status gives immunity to physical damage. The player can apply it using Aerith's level 2 limit break, Planet's Protection.
A variety of enemies can also apply this status to themselves. Some, such as Ghosts, can alternate between it and 
Reflect, forcing the player to use both physical and magical attacks to defeat them.
Final Fantasy VII Rebirth
Shield nullifies physical damage for around 80 seconds. Magic damage still works. It is applied with Aerith's Planet's Protection limit break.
Final Fantasy X
Shield status bestowed with an aeon's Shield command cuts damage to ¼ but the aeon's Overdrive gauge will not fill.
Final Fantasy Crystal Chronicles: My Life as a Darklord
Final Fantasy Record Keeper
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.