Luck is a magical buff applied by various spells, items, and a single Bless effect. No one is innately Lucky. Luck of this sort extends the holder's battlefield life, by making any hit that would kill them miss 75% of the time.
Luck can only save lives. Luck cannot be applied to Inanimate beings, and it can't be applied to the technically-living Undead (such as Ghouls).
Luck also works against spells, even area-of-effect (AOE) ones. However, instakill effects generally leave no room for Luck:
Luck is a Glamour effect. The vast majority of Lucky items require Glamour to make, though 11 mages can also make one. There are four Luck-bestowing spells: Personal Luck (Thaumaturgy 1), Luck (Thaumaturgy 3), Group Luck (Thaumaturgy 5), and Battle Fortune (Thaumaturgy 8). The first two require just 1, Group Luck requires 2, and Battle Fortune requires 4 and 2 (or 3 and 3).
Astral has a knockoff version of Luck, known as Fool's Luck, provided by the Spell Focus. On top of saving lives, Fool's Luck works against every attack that does more than one point of damage, but it doubles damage instead of negating it every second time it kicks in.
Not to be confused with Twist Fate, nor the Luck/Fortune Scale.