The general concept is limiting the target's Movement Speed for a short time. This is most useful to prevent the enemy from escaping their doom, or from chasing down a hero willing to run with their life. It can also be used to kite the enemy without leaving combat, giving the advantage and keeping the victim at safe range.
Chill is a special form of Slow, triggered by Cold attacks and Cold-related skills only. In both games, any Cold damage may trigger Chill. Chilled targets are blue in color and move (and sometimes attack) much slower. Unlike Freeze that paralyzes the target completely, Chill only slows the victim down. General rule is that a Slow effect applied by a Cold skill will most likely count as Chill. Another way to determine it is to look at the color: blue or light blue coverage (like that of Freeze effect) will mean a definite Chill.
In Diablo II, Slow can happen due to Cold attacks, Holy Freeze Aura, Decrepify Curse, Clay Golem's slowing attack, items with "Slows Target By X%" and Cold Skills. Even monsters that are immune to Cold can be slowed. Cast rate is not affected by Slow. Slowing has two main types: non-elemental Slow and Chill.
- First is the non-elemental slowing. It works as a simple "speed debuff". Some unique and set items have the "Slows Target By X% mod; and there are skills with this effect like Decrepify, Holy Freeze and Clay Golem's attack. Notable that Holy Freeze's slow is not a simple cold and cannot be resisted with items with the Cannot Be Frozen modifier and Cold Immune monsters are affected by it as any other monster, only the periodic (and very low) cold damage will be resisted. Items and skills using this effect stacks with a cap about 85%.
- Second is Chill, when Cold damage causes the slow effect. It can be resisted by players with items with the Cannot Be Frozen modifier, and by monsters via cold immunity. Its duration is also reduced by cold resistance, and items with Half Freeze Duration modifier. Every cold damage have one type of this effect. Chill is considered less potent than the more powerful Freeze, but unlike common Slow, it can reduce the Attack Speed. Any Freeze effect on Act Bosses, Super Unique Monsters and Unique Monsters, as well as other Player Characters, will work only as Chill. This will generally slow enemies by 50% in Normal, 40% in Nightmare, and 33% (25% for Undead) in Hell, although it is less effective on some monsters (and totally ineffective on a certain monster). A Chilled enemy has a chance to be shattered when killed, leaving no Corpse. Thawing Potion instantly clears the Chill effect on players.
In Diablo III, Slow and Chill are again separate effects. Chill can happen only due to Cold skills and Cold damage, but always has a small chance to trigger on non-player targets, even if no equipped items have the chance to Slow on hit affix. Many Skill Runes that change the damage type to Cold also add a chance, or a guaranteed Chill or Freeze. Consequently, being immune to Cold damage is guaranteed to protect the target from Chill, but has no effect on non-elemental Slow. Slow is also common, and affects both players and monsters. For example, Spiders leave pools of web that slow the players stepping on them by 60%.
There are three types of Slow: 30%, 60% and 80%. They stack, but cannot go over 80% at any time. The monsters' CC resistance does not change Slow duration, but reduces its potency, to a minimum of 5%. Some effects, like Slow Time, reduce not only the enemy's movement speed, but the attack speed as well.
Elites also have innate 25% reduction to Slow effects on movement speed and 65% reduction to attack speed penalties. The reduces duration of control impairing effects affixes on players' items reduce the duration, but players cannot reduce the potency of Slow effects.
Chilled enemies do not give a chance to shatter anymore (but Critical Hits from Cold attacks do). Chill and Slow count as single type of CC for the purpose of CC resistance, and most effects that require the target to be Slowed to trigger do so regardless of whether it is a common Slow or Chill. However, in some very rare circumstances, such effects may specifically require the target to be Chilled: Cold Blooded, Iceblink and Frostburn are examples of that.
Immobilize can be considered the extreme variant of Slow, just as Freeze is an extreme degree of Chill. However, Slow and Immobilize effects do not stack, do not override each other, and are not interchangeable for all purposes.
Chill returns in Diablo IV, as a Sorceress class mechanic. Unlike the previous incarnation, it stacks, until it turns the foe Frozen at 100% slowdown, and after death, shatters them and leaves no corpse.