Resist
A creature that has resistance takes less
damage from a specific damage type. For
example, a creature that has resist 10 fire
takes 10 less damage whenever it takes fire
damage.
Against Combined Damage Types: Your
resistance is ineffective against combined
damage types unless you have resistance to
each of the damage types, and then only the
weakest of the resistances applies. For
example, if you have resist 10 lightning and
resist 5 thunder and an attack deals 15
lightning and thunder damage to you, you take
10 damage, because the resistance to the
combined damage types is limited by the lesser
of the two resistances.
Not Cumulative: Resistances against the same
damage type are not cumulative. Only the
highest resistance applies. For example, if you
have resist 5 cold and then gain resist 10 cold,
you have resist 10 cold, not resist 15 cold.
Similarly, if you have resist 5 cold and then
gain resist 2 to all damage, you still have resist
5 cold, not resist 7 cold.
Published in Monster Manual 2