Attack Modifier

Attack Modifiers are modifiers which apply an effect to a unit's basic attacks. These effects can widely vary, from healing, to damaging or even disabling effects. Most of these modifiers have their own rules. For example, some modifiers may not stack with others, some may fully stack, some may be for melee units only and others for ranged or both. Attack modifiers can be active, in which case they require to be used manually, though most have an autocast option, but they also can be passive, proccing, or having a chance to proc on each attack.

Stacking
There is no set rule of how attack modifiers stack, except for the group of unique attack modifiers. In most cases, an attack modifier does stack with another attack modifier.

Common attack modifiers
There are many attack modifiers which are seen very commonly in the game. The only difference between them are their numbers and how they are activate, some being applied on each attack, others being chance based.

Abilities which simply grant or reduce attack speed (e.g. or ) or attack damage (e.g.  or ) are not attack modifiers in that sense, as they directly enhance the attack, instead of granting a modification.

Critical strike
Critical strike causes the attacks to deal multiplied damage. Most critical strikes are chance-based, but not all. This modifier can be used by any unit. Critical Strikes fully stack with other attack modifiers. However, they do not fully stack with themselves. Multiple sources of critical strike have all their own proc chance. But if multiple would proc on the same attack, only the one with the higher multiplier is applied.
 * Examples of critical strike are, and.

Cleave and splash
Cleave causes the unit's attack to deal damage in a circular area in front of the unit. The damage is based on the unit's attack damage. Cleave can only be used by melee units. Cleaves fully stack with other attack modifiers. Multiple sources of cleave on the same unit work fully independently of each other. Each source applies its full damage within its area, without interacting with the other cleave sources.
 * Examples of cleave are, and.

Splash works similar to cleave. It causes the unit's attack to deal area damage, but this area is rather a circular area centered around the attacked unit. Splash can only be used by ranged units. Splash fully stacks with other attack modifiers. Multiple sources of splash on the same unit work fully independently of each other. Each source applies its full damage within its area, without interacting with other splash sources.
 * Examples of splash are and.

Bash
Bash causes the unit's attack to stun the target and in some cases to also deal extra fixed damage. As of now, all bashes are chance-based. Bash can be used by any unit. Bash fully stacks with other attack modifiers, except with other bashes. Multiple sources of bashes do not stack at all. The later acquired bash has absolutely no effect. The only exception is, which fully stacks with any other bash.
 * Examples of bash are, and.

Lifesteal
Lifesteal causes the unit's attack to heal the attacking unit. In any but one case, the heal is based on the unit's attack damage. Lifesteal can be used by any unit. Lifesteal can come in the form of a regular attack modifier, in which case it fully stacks with any other attack modifier, including other sources of lifesteal, or in the form of a unique attack modifier, in which case it does not stack with other sources of unique attack modifiers.
 * Examples of lifesteal are, , and.

Mana break
Mana break causes the unit's attack to burn a portion of the target's mana and deal damage based on the amount of mana burned. Mana break can be used by any unit. Mana break can come in the form of a regular attack modifier, stacking with all other attack modifiers, or in the form of a unique attack modifier, stacking with all attack modifiers, except with other unique attack modifiers. Multiple sources of mana break do not stack at all (the source of mana break which is oldest overrides the others), regardless of which type it is.
 * Examples of mana break are, and.

Other modifiers
There are many modifiers with unique effects. Most of them are specific to a hero, unit or item. Most of them fully stack with every other attack modifier. Although these abilities have all unique effects (i.e. none of these abilities appear on more than one hero), these are not Unique Attack Modifiers.

Active attack modifiers
Active attack modifiers are modifiers which must be actively used on each attack to apply its effects. They also can be set on autocast, causing each attack to apply the effect if the hero can support their costs. Some of these are Unique Attack Modifiers.

When set on autocast, they still count as a regular attack. When used manually, they partly count as a spell cast. This means, unlike regular attacks, they do not cause any aggro from lane creeps. However, they do not count as spells in any other scenario, meaning they cannot be used when attacking is restricted and do not proc anything which triggers on spell cast.

Orb walking
Issuing an attack command on an enemy hero causes all nearby enemy creeps and towers to switch their aggro to the attacking unit. This is not the case for casting spells on enemy heroes. Orb-walking is the act of manually casting active attack modifiers like other active abilities, instead of using autocast and right-click or auto-attacks. When used manually, the attack is partially treated as a spell cast. This causes the hero to not draw aggro from nearby enemy creeps, making it possible to harass an enemy in the lane without the creeps and even towers refocusing their attacks on the harassing hero as they normally would when using regular attacks or autocast. However, the effect still counts as an attack and not a spell cast in regards of on-cast effects. It still uses the hero's attack speed and animations and still puts the attack on cooldown normally.

Origin of the term "orb"
In Warcraft 3, unique attack modifiers are called "orb effects". This is due the items which grant the attack modifiers commonly being orbs (for example, Orb of Venom, Orb of Darkness, Orb of Lightning, Orb of Corruption, etc). A characteristic of these orb effects is that they do not stack with each other. The items being portrayed as orbs serves as a metaphor, as stacking real sphere-shaped objects is very difficult to do. Since DotA is a custom map on Warcraft 3, it made use of the abilities of these orb items, and the term "orb" was used to describe that attack modifiers based on those items do not stack. Therefore, unique attack modifiers are referred to as "orb effects" in DotA, just like how they are in Warcraft 3. The term was changed to "unique attack modifier" in Dota 2, while "orb effect" was dropped, however, many players which came from DotA still refer to these modifiers as orb effects. The term "orb walking" originates therefore from Warcraft 3, describing the same technique as described earlier.

Later usage of the term
A later usage of the term "orb walking" refers to using attacks or active attack modifiers to achieve stutter stepping. "Stutter stepping" is the act of cancelling the backswing animation of an attack, allowing the unit to move between attacks, following the target, instead of standing and waiting for the backswing to finish. This old technique, which originated from Starcraft and carried over to Warcraft and therefore also to DotA, was commonly used together with the orb-walking technique. This is because of how manually casting active attack modifiers behaves, as compared to regular attacks. During the cast time of a manually cast active attack modifier, the player can spam the move command, which does not cancel the ongoing cast, but the first move command after the cast point is reached cancels the backswing, so the hero moves almost immediately after the attack launched. However, doing that during a regular attack does cancel the regular attack, causing the unit to move immediately, so the player must time the move command after the attack launched. This means stutter stepping was much easier to do with casting active attack modifiers, as timing for the move command was not required, allowing for much more effective stutter stepping. Due to this, orb walking and stutter stepping were very commonly used together, so commonly that players started to call the move just orb walking, even though it was orb walking and stutter stepping together. This resulted in the community to commonly refer to stutter stepping as orb walking, even though stutter stepping can be done with regular attacks as well. The term stutter stepping became less known as a result. However, outside of Dota, the technique is still called stutter stepping, which may cause some confusion when mentioning the technique in other communities (like Starcraft, League of Legends, Command and Conquer and other RTS and Dota-like games), as they most probably only know it as stutter stepping.

Unique attack modifiers
Unique attack modifiers (formerly known as Orb Effects) are a group of attack modifiers with certain stacking rules, which only apply to those. This means that other attack modifiers fully stack with them without problems.