Dispel

A dispel is the term used for a forced removal of status effects.

Dispelling itself is not granted explicitly by any effects or spells, it is instead granted as a secondary effect to other abilities. For instance: A Purge dispels, slows, and does damage to summons; a Cyclone dispels and grants invulnerability; and Mirror Image type abilities dispel, disjoint, give invulnerability, hide the unit and spawn illusions.

Dispels can be categorized into three major categories by the type of status effects they remove. Normal dispels will remove debuffs from allies and buffs from enemies, dispels from magic immunity will remove both positive and negative status effects, lastly strong dispels can only be used on allies but in return they are the strongest type of dispel and can even remove stuns.

Normal dispels
The most common type of dispel. When targeted on an ally or self, only negative status effects (debuffs) will be removed. When targeted on an enemy only positive status effects (buffs) will be removed. Normal dispels can not remove stuns.

Cyclones
In addition to being dispelled, units affected by a Cyclone are made invulnerable and disabled for a set duration.

Purges
In addition to a dispel, a Purge applies a slow on affected units (if they are enemies of the caster) and, if cast on a summoned unit, deals damage.

Mirror Image type abilities
A Mirror Image type ability makes illusions of the caster, disjoints projectiles, grants a small period of invulnerability, and dispels certain status effects.

Other sources of dispel
These abilities apply a normal dispel to their target(s).

Dispels granted by magic immunity
Dispels granted by magic immunity differs from normal dispels in that they often remove positive buffs in addition to negative ones.

Strong dispels
Strong dispels differ from normal dispels in that they remove the vast majority of negative buffs and can even remove stuns. There are no strong dispels in the game that can be targeted at an enemy, as such they will only remove negative status effects (debuffs) on allies.

Note: As of 6.82: Dark Pact, Kraken Shell, Aphotic Shield, Press the Attack and Time Lapse DO NOT dispel Poison Nova, Heaven's Halberd, Ensnare and slow from Eye of Skadi