Denying experience points is a common strategy in DotA. This means killing an allied creep with low hit points left before your opponents do. Denying a creep will reduce or negate the experience gain for enemy heroes.
A denied creep will give 66% of its normal experience value to melee enemy heroes and 33% for ranged enemy heroes. A creep can only be denied if it has less than 50% HP.
Aside from creeps, buildings can also be denied. Defense towers are often denied to prevent the opposing team from getting a huge amount of gold. However, buildings must have 10% or less HP before it becomes deniable.
Under certain circumstances, an allied hero can also be denied. When a hero is affected with a Damage Over Time skill and is under 25% of his maximum HP, allied heroes can attack and kill him in order to prevent the opposing team from getting gold and gaining XP.
The following skills will make a hero deniable: Doom, Shadow Strike, Shadow Word and Curse of the Silent. Likewise, denying will trigger these skills in the hero: Sadist, Blood Bath, Flesh Heap, and Necromastery.
Using these skills to deny a creep will give absolutely no XP to enemy heroes:Dark Ritual, Death Pact, Conversion and Infest.
You can type -di or -cson any time in the game to activate the deny record chart. The deny record chart shows you how many lanes creeps you killed and how many you denied.