The Knife is the default melee weapon in Killing Floor and Killing Floor 2. You begin the match with it in your inventory and it has no Weight and it cannot be dropped or sold.

Killing Floor

"Military Combat Knife."
The Knife has no weight, and equipping it lets you run at the fastest speed possible.

The knife itself, is very weak but has a fast melee attack state, allowing you to temporarily stun a specimen by hitting it repeatedly. This can save your life in the higher levels. However, Fleshpounds, Husks, Scrakes, Bloats and the Patriarch are not stunnable with the use of a knife.

Kills with the knife count towards leveling the Berserker, and Berserkers receive damage bonuses with it, even though it is available to all Perks by default.

Killing Floor 2

Each Perk now has their own Knife. They all function the same, but have different names and appearances. Additionally a knife will only gain XP for the Perk that owns it. If you begin the game as one Perk and then switch to another mid-match, your knife will not automatically switch over like your Grenade will, so kills with it will still only gain XP for your original Perk.

Unlike the first game, equipping your knife does not make you run faster.

Until the Treacherous Skies update, the Survivalist did not have their own knife and instead used the Support's Machete.

Each knife can have it's own separate Weapon Skins.