Hits against any target can potentially cause a Stun on that target, depending on the damage dealt. Stunning a target interrupts their current action and prevents them from taking actions for a short time. There are two types of Stun:

Light Stuns last a fraction of a second but can be inflicted frequently. Any Hit has a chance to cause a Light Stun. The chance is based on the damage dealt, up to 100% base chance for Hits that deal 100% of the target's maximum Life. Chances lower than 15% are treated as 0%.

Heavy Stuns occur when a target's Stun bar is filled and last multiple seconds. Hits cause Heavy Stun buildup based on the damage dealt. Players and their Minions usually cannot be Heavily Stunned, but players can receive Heavy Stun buildup where specifically mentioned while taking specific actions (such as raising their Shield, Parrying with a Buckler, or riding a mount).

Player Physical damage and Melee Damage each have 50% more Light Stun chance and Heavy Stun buildup. These bonuses are multiplicative with each other.
Monster Physical damage and Melee Damage have 100% more and 33% more Light Stun chance and Heavy Stun buildup respectively. These bonuses are multiplicative with each other.

Any Hit has a chance to Light Stun the target, interrupting their current action and preventing them from taking actions for a fraction of a second. The chance is based on the damage dealt, up to 100% base chance for Hits that deal 100% of the target's maximum Life. Chances lower than 15% are treated as 0%.

Physical damage, and player (but not monster) Melee Damage, each have 50% more Light Stun chance. For players, these bonuses are multiplicative with each other.

Heavy Stuns occur when a target's Stun bar is filled, interrupts the target's current action and prevent them from taking actions for a few seconds. Hits cause Heavy Stun buildup based on the damage dealt. Players and their Minions usually cannot be Heavily Stunned, but players can receive Heavy Stun buildup where specifically mentioned while taking specific actions (such as raising their Shield, Parrying with a Buckler, or riding a mount).

While an enemy is Heavy Stunned, they cannot act and count as Immobilised. It will also be harder to Heavy Stun them again for a short time afterwards.

Physical damage, and player (but not monster) Melee Damage, each cause 50% more Heavy Stun buildup. For players, these bonuses are multiplicative with each other.

Slams are Melee Attacks that cause damaging areas of effect, rather than Striking enemies with your weapon directly. Damage from Slams is still Melee damage.

A Strike is a Melee attack that directly hits with weapons or body parts. Slam attacks do not count as Strikes.

Swords are Melee weapons that can be One-Handed or Two-Handed. Swords require Strength and Dexterity to equip.

Sword Attacks are commonly related to elemental damage.

Maces are Melee weapons that can be One-Handed or Two-Handed. Maces require Strength to equip.

Mace Attacks are often Slams or slow Strikes that deal Physical or Fire damage.

Axes are Melee weapons that can be One-Handed or Two-Handed. Axes require Strength and Dexterity to equip.

Axe Attacks commonly involve throwing your axe and/or inflicting Bleeding.

Daggers are One-Handed Melee weapons that require Dexterity and Intelligence to equip.

Dagger Attacks are commonly related to ambushing or debilitating enemies. Some blade-related Spells also require a Dagger.

Claws are One-Handed Melee weapons that require Dexterity to equip. Claws can be Dual Wielded with another Claw, but cannot be combined with other equipped items in the off hand.

Claw Attacks are commonly fast and cause Bleeding.

Talismans are Two-Handed Melee weapons that require Strength and Intelligence to equip.

Talismans allow you to use Shapeshifting Skills associated with the Talisman's current form, and grants a basic Attack for that form. You can change the Talisman's form at any time.

While you have a Talisman in your active weapon set you will Shapeshift into its associated form. As a result, Skills that cannot be used in a Shapeshift form cannot be used with Talismans. Triggered Skills and Persistent Skills will continue to function.

Quarterstaves are Two-Handed Melee weapons that require Dexterity and Intelligence to equip.

Quarterstaff Attacks often focus on high mobility in combat.

Spears are One-Handed Melee weapons that require Strength and Dexterity to equip. Spears cannot be dual wielded.

Spear combat is often a mix of both ranged and Melee, as many spear skills enable you to throw your spear.

Flails are One-Handed Melee weapons that require Strength and Intelligence to equip. Flails cannot be Dual Wielded.

Flail Attacks tend to reward careful placement and long wind-up times with devastating effects.

Thorns damage is a kind of Hit Damage you can deal. Thorns damage is not Attack damage or Spell damage and is not affected by modifiers specific to those.

If you have Thorns damage, you inherently Retaliate against Melee Attack Hits, dealing your Thorns damage to the enemy that Hit you.

Some skills and other effects may also deal your thorns damage to enemies at other times.

Close Range is anywhere within 2 metres of your character. Normally, Melee Attacks you make are in Close Range.

Elemental Surges are consumed when you use a non-Melee Projectile Attack to suffuse the Projectiles fired by that Attack, causing them to trigger a Surging Blast when they reach the end of their flight.

Your weapon can have a maximum of 6 of each type of Surge active by default. Surges last 15 seconds. Surges are specific to your current weapon, so do not affect non-weapon damage and are not carried over if you weapon swap.

Projectiles which Split or Fork do not gain the benefits of Surges.

Whirlwinds Blind and Slow the movement speed of enemies within them. If their creator crosses the edge of the Whirlwind it collapses, damaging and Knocking Back enemies caught inside. The collapse deals Melee damage.

Trying to create a Whirlwind that would overlap with an existing Whirlwind instead moves the existing Whirlwind and grants it a stage, making it larger and more damaging.

A Whirlwind that overlaps an allied Elemental Ground Surface takes on that element, gaining 50% of damage as the corresponding type and applying the Ground Surface's debuff to enemies inside the Whirlwind for 8 seconds.

This allows your Thorns to Retaliate against any kind of Hit dealt to you by enemies, rather than just Melee Attacks, except that you can never Retaliate against the enemy's Thorns damage.