The term resist means that a player, mob, or NPC was not, or only to a lesser extent, affected by a spell or elemental attack. Full resists completely negate the effect and have the same effect as Immune. Partial resists only reduce the damage received by a percentage; these can only occur with non-binary spells.
There are two sources for spell resists. Firstly, the level difference between the caster and their target determines the spell hit chance. A spell which misses its target results in a full resist, whereas partial resists can not occur. Spell hit rating decreases the chance of getting a resist due to this difference. Secondly, Resistances increase the chance to resist a spell. They can cause both full and partial resists, and can be overcome with spell penetration (and, limited to binary spells, spell hit rating). A full resist due to level difference cannot be distinguished from a full resist due to resistances.
There are also some negative spell hit effects in the game, such as the Balance of Power and Unbreakable Will. The former decreases the total spell hit chance of the attacker, whereas the latter only affects the spell hit chance of specific abilities.
Resists are the magical counterpart of Miss, Dodge, and Parry. Even though the Armor attribute and Defense skill generally affect the ability to mitigate physical damage, they are not considered resistances. However, there are some border cases; for example, the Hunter's Arcane Shot can be partially resisted, but a full resist is shown as a miss in the combat log. Some spells, such as Chaos Bolt, cannot be resisted.