Multistrike move
Multistrike moves are moves that strike an opponent more than one time in the same turn. Most multistrike moves will hit the opponent 25 times in a turn, but some work differently. Multistrike moves generally have low base power and imperfect accuracy to compensate for the fact that they hit multiple times. Bide, Counter, and Mirror Coat only take the final hit of a multistrike move into consideration.
Effect
Generation I
If an opponent is knocked out by a multistrike move, the game does not state how many times the attack hit.
Although only the first strike can be a critical hit, each successive one will deal the same amount of damage. In this generation only, multistrike moves end instantly upon breaking a substitute. Bide and Counter deal damage based on the last strike alone.
Generation II
If the target faints, the number of strikes is shown afterward.
Each strike has an independent chance of being a critical hit.
Multistrike moves can now continue hitting even after breaking a substitute, then dealing damage directly to the targeted Pokémon. If a multistrike move activates a target's Focus Band, no subsequent hits can knock it out.
If the user is holding a King's Rock, the chance of causing the target to flinch is independent of how many times the move hit (i.e., always ~12% in Generation II).
Generation III
The number of hits is now shown before the target faints.
Color Change will activate after the first hit of a multistrike move, possibly changing the type effectiveness of the remaining strikes.
Each strike from a multistrike move has an independent chance of activating an Ability that activates on taking damage, and each strike of a multistrike move that makes contact has an independent chance of activating an Ability that activates on contact. If a Pokémon using a multistrike move falls asleep or faints between strikes, such as due to Effect Spore or Rough Skin, the move will end immediately. In every Generation III game except for Emerald and XD: Gale of Darkness, due to a glitch, if a Pokémon uses a multistrike move other than Triple Kick against a target with an Ability that inflicts a status condition on contact, and the move's user triggers this Ability with the move's final strike, there is a 1% chance that the target also becomes afflicted by the status condition inflicted by their Ability.
If the user is holding a King's Rock, each strike now has an independent chance of causing the target to flinch.
Shell Bell activates after each strike, considering the damage from that strike only.
Generation IV
If a multistrike move activates a target's Focus Sash, no subsequent hits can knock it out. Each strike has an independent chance of activating a Razor Fang. Shell Bell activates after the last strike, considering the damage from all of the strikes.
Pokémon with Skill Link will always hit five times when using a 25 hit multistrike move.
Generation V
The number of strikes is now shown after the target faints.
The Focus Sash, Focus Band, and Sturdy need to activate for each individual strike. A Focus Band can still activate repeatedly to prevent fainting, but each chance is independent.
Color Change now activates after the last hit of a multistrike move, so type effectiveness will not change during the attack.
Each individual strike from a physical multistrike move will activate Weak Armor. Each individual strike from a Darktype multistrike move will activate Justified; however, a Grasstype multistrike move will only activate Sap Sipper once. Each individual strike from a contact multistrike move will activate a Rocky Helmet.
Each strike has an independent chance of activating Stench (but this does not stack with the Razor Fang or King's Rock).
Items like Cell Battery, etc. activate on the first strike and are consumed in the first strike itself.
Generation VI
Parental Bond turns most singlestrike damaging moves into multistrike moves that hit twice per use.
If a contact multistrike move is blocked by Spiky Shield or King's Shield, the negative effect will only apply once.
Generation VII
Each individual strike from a multistrike move will activate Stamina.
Generation IX
If a Loaded Dice is held, moves that hit 25 times now hit 45 times; Triple Axel and Triple Kick will only have one accuracy check; Population Bomb will also only have one accuracy check, and it will hit 410 times.
List of multistrike moves
Fixed number of multiple strikes
Several multistrike moves hit an opponent a specific number of times per use. Most of these moves always hit exactly twice, but Surging Strikes and Triple Dive always hit exactly 3 times.
These moves are typically less common than multistrike moves that hit 25 strikes per use, with several of them being signature moves. They generally have higher power than multistrike moves that hit 25 strikes per use, as they only hit twice. Moves affected by Parental Bond become this type of multistrike move.
Uniquely for a multistrike move, Dragon Darts can hit different targets with each of its strikes.

Variable number of strikes
Most multistrike moves will hit an opponent 2, 3, 4, or 5 times for each use. These moves are more likely to hit 2 or 3 times than 4 or 5, but will always hit 5 times when used by a Pokémon with the Skill Link Ability.

In Generations IV and earlier, moves in the below list will hit on average 3 times, provided that the move does not miss. From Generation V onward, the average number of hits is now 3.1, provided that the move does not miss.

 Average power is equal to the move's base power per hit multiplied by the average number of hits, provided that the move does not miss. To take accuracy into account in the average power, multiply the two together (possibly after making adjustments for accuracy and evasion modifiers).
 Prior to Generation IX, if Battle Bond is activated, Water Shuriken will always hit three times, and its base power will increase to 20.
Accuracydependent multiple strikes
Triple Kick and Triple Axel can hit up to three times per use, while Population Bomb can hit up to ten times. An accuracy check is performed before each strike to determine if the move continues. From Generation V onward, if the user has Skill Link, only the initial accuracy check is performed.
For Triple Kick and Triple Axel, the power of each subsequent hit is greater: Triple Kick has 10 power on its first hit, 20 on its second hit, and 30 on its third hit; Triple Axel has 20 power on its first hit, 40 on its second hit, and 60 on its third hit. Population Bomb has a power of 20 for each hit.

In the table above, effective power is average power with all three accuracy checks taken into account (modifiers to accuracy and evasion, including taking aim and No Guard, are not considered).
Partydependent multiple strikes
This section is incomplete. Please feel free to edit this section to add missing information and complete it. 
Beat Up uniquely hits a number of times equal to the number of Pokémon in the user's party, excluding those who are fainted or who have a nonvolatile status. Its base power and damage mechanics have varied between generations.
In Generation II, each strike of Beat Up deals typeless damage with a base power of 10, but uses each individual team member's Attack stat and level to calculate damage; as a result, its damage is unaffected by any changes to the user's Attack stat (such as Swords Dance). Because it deals typeless damage, Beat Up receives no benefit from Sametype attack bonus and ignores Wonder Guard.
Starting in Generation V, Beat Up deals Darktype damage, benefits from STAB, and interacts normally with Wonder Guard. Instead of using each party member's Attack stat to calculate damage, Beat Up only considers the user's Attack, meaning any change to the user's Attack through moves like Swords Dance now applies normally. Instead, party members' base Attack stats are used to calculate their strikes' base power, with the new formula:
For example, a Weavile using Beat Up with active team members Shuckle and Conkeldurr would make three strikes with base powers of 17, 6, and 19, respectively.
Trivia
 Double Slap is the only multistrike move with "double" in its name that can hit 25 times, though its Japanese name おうふくビンタ Round Trip Slap is more accurate.
This article is part of Project Moves and Abilities, a Bulbapedia project that aims to write comprehensive articles on two related aspects of the Pokémon games. 