Quick Melee

In older games, melee weapons were like any other weapons, if you wanted to use it, you had to switch to it. Recent games have it being a simple player action. With a press of a button, the player character will use the melee weapon, (or the butt of the gun) and will immediately switch back to the original weapon he or she was holding.

This usually comes in two versions. The first version of this system would involve the player character swinging the weapon he or she is holding. The other version, and newer one, involves the player immediately whipping out a knife or some similar melee weapon, and switching back to the weapon they originally had out. Usually, this second variation is an instant kill. On the flipside, if there are dedicated melee weapons, the developer may encourage their use by giving a Necessary Drawback to this, such as making the quick attack fast but weak while the dedicated melee weapon is stronger. Halo comes to mind, where the Energy Sword and Gravity Hammer largely outperform the quick Pistol-Whipping.

While the first variation is usually accepted by most gaming circles, the second variation usually leads to a Broken Base, with one camp thinking of it as just another game mechanic, and another thinking it is just making the games rely more on twitch reflexes than actual skill.

See also, Bayonet Ya.

Examples of Quick Melee include:

Gears of War Features this, and if you have out the Lancer chainsaw bayonet...
Left 4 Dead: Players could melee with their firearms or whatever they were holding to push back or even kill zombies.
The oldest known example of such a key is Duke Nukem 3D, where Duke could kick with his left foot on the press of a button. As the kick does not stun most enemies, and being near enough to use it makes it impossible to dodge attacks, it's mostly used for breaking vent covers. Unpatched versions (including the shareware) had a bug that allowed both this key and the normal melee weapon (Duke's right foot) to be used at the same time, resulting in Duke presumably running around doing Hopak.
Halo: With a press of the button, whatever weapon the player was holding would be used as a club. Damage output would depend on the weapon used.
Call of Duty: Before the Modern Warfare series, the player would use the butt of a gun to hit an enemy.
Modern Warfare, World At War, and Call of Duty Black Ops use this and are arguably the ones who caused the trend of the second version, or "quick knife" in recent shooters. The first Modern Warfare brought it to the scene; World At War added the bayonet, which gave the user a longer melee range; Modern Warfare 2 slowed the knife speed but brought in both the tactical knife, which allowed quicker melee, and the Commando perk, which gave players a longer melee range (and was arguably a Game Breaker). 3 dropped Commando and removed the "lunge" from knifing, but kept the tactical knife.
The 2010 version of Medal of Honor had a quick knife for the US side, and a quick axe for the OpFor side.
Bad Company 2: Very similar to Call of Duty's "quick knife", albeit a bit slower.
Battlefield 1942: In the Road To Rome expansion pack, the engineer had a bayonet, which when selected, would be used when the player hit the right mouse button, rather than the typical "zoom" feature their rifle would normally have.
Battlefield 3 has an interesting variation. There is the quick knife, which kills in two swipes, or the takedown, which takes a few seconds to do, but rewards the player with dogtags and a unique animation. The knife can be wielded like any other weapon.
Home Front: Exactly like Call Of Duty's knife.
Shattered Horizon had a Quick Pickaxe. Early versions of the game had a retractable bayonet in the assault rifle.
Singularity has the quick-knife in the first part of the game. After the player receives the TMD, it's replaced with the Impulse attack, a burst of energy capable of dismembering most human enemies at point-blank range.
Metro 2033: Some weapons could be used as melee weapons, such as the shotgun. If the gun had a bayonet on it, the player would stab with the bayonet instead.
Republic Commando If you have a rifle (possibly with an attachment), pressing the appropriate button will make you use a wrist blade. Otherwise, you just hit the enemy with the gun.
Melee combat in Wet is realized this way. Basically, Rubi's default weapons are her guns, but she can pull out her katana and slash the enemy in front in a single motion, which is triggered with a controller button.
Resident Evil 4 does this with the knife, a good thing since said knife is decently strong in this game and there's too many mooks around for you to be wasting ammo.
In Max Payne 2, both characters have a "secondary weapon" button, offering the player a choice of throwing grenades, Molotov cocktails or simply striking enemies with the butt of their gun.
In Second Sight, if one is close enough to an enemy and presses the attack button, the protagonist will strike the enemy with the butt of his gun (or kick them if they're prone).
Red Orchestra: Ostfront 41-45: You can bash enemies with your gun with the press of a button. Some classes also come with either an attachable bayonet or a gun that always has a bayonet.
Resistance has the quick gun-melee variation. The upcoming Gaiden Game Burning Skies replaces it with a touch-activated quick fire-axe.
First Encounter Assault Recon also does the first variation. The third game replaced it with the knife.
Day Of Defeat: The original game allowed players to hit others with the butt of their gun. Source does away with this, but adds in a punch feature that players can perform when using an SMG.
In Borderlands all the characters have a unique melee weapon (Lilith zaps them with some sort of energy, Mordekai uses a sword, Brick uses a lead pipe, or fists when berserking, and Roland uses a knife ), plus some pistols have attached blades which do bonus melee damage when they're used.
Killzone has a button to beat an enemy with the butt of your rifle, though it only inflicts a small amount of damage and some staggering. You do have an instant-kill knife, but you have to manually equip it.
In Zone of the Enders, the "Attack" button fires a projectile shot by default, but if you lock on to an enemy and fight them at close range, it will automatically execute stronger melee attacks instead.
All characters in Metal Slug will automatically stab instead of shooting if the enemy is too close. Unless that enemy is a vehicle.
- Same goes for Alien Hominid, a similarly styled game.
In Deus Ex Human Revolution you can perform a melee takedown attack on an enemy at close range (tapping the button to chock them or deliver a Tap on the Head with your Artificial Limbs or holding it to stab them to death with your Blade Below the Shoulder). Two if you get a reflex booster aug.
- Zero Punctuation complained about this trope in his column regarding the above, saying how he misses dedicated melee weapons and how different games had their own iconic melee weapon (e.g. the crowbar from Half Life).
Played With in Grand Theft Auto IV. Getting extremely close to an enemy and firing will instead have your character hit the enemy with the weapon, though there's no actual dedicated button to Quick Melee. The game also has normal melee weapons you have to switch to.
Saints Row has this in addition to "normal" melee weapons. There's a different animation for each weapon, and particularly in The Third most of them have you hitting the guy you're fighting in the nuts.
Gotham City Imposters has it, very similarly to Call of Duty's.
A variation in Brink. If you have a two handed weapon out, like a rifle, your character will attack with the butt of it. However, if you have a one handed weapon out, like a pistol, you will attack with a knife. Knives do more damage, but don't stun, which two handed melees do.
Shows up in all three Mass Effect games. The first simply has you whack your target with your gun if you press the fire button while you're at melee range. 2 adds a dedicated melee button. 3 slightly changes it up - tap the melee button for a quick pistolwhip/buttstroke, or hold down the button to unleash a powerful hit from your Blade Below the Shoulder, or a biotic-enhanced punch for an Adept or Vanguard.