Resistance

RESISTANCE is a magical ability to become partially invulnerable to a certain type of damage. Natural abilities, spells, and potions are common causes of Resistance.

A creature that is resistant to a certain damage type will take half damage from that source. Round down. So, if an adventurer who drank a Fire Resistance potion takes 8 Hit Points of Fire Damage, they only mark off 4 Hit Points. 7 Hit Points of Fire Damage become 3 Hit Points of damage.

DnD categorizes all Hit Point damage into certain types: Fire, Cold, Poison, Necrotic, Radiant, Piercing, Slashing, Bludgeoning, Thunder, Radiant, Psychic, and the list goes on…

We usually only mention damage types when there is the possibility of resistance.

IMMUNITY is leveled-up Resistance. A creature with Immunity to a type of damage takes no damage from that one source. Certain creatures have Immunity to a certain type of damage; Fire Elementals are Immune to Fire Damage. Additionally, some magic items, spells, or potions can grant temporary Immunity.

VULNERABILITY is the evil cousin of Resistance. A creature with Vulnerability takes double damage from that type of damage.