A Saboteur is a creature with a triggered ability upon dealing combat damage to a player. The nickname is derived from the creatures having to sneak behind enemy lines to generate an effect unfavorable for the opponent. Prolific implementation of this mechanic include Specters who cause the opponent to discard cards from their hand as well as Ninja with various saboteur effects but the ability Ninjutsu to make it easier for them to deal damage to the opponent.
Another high profile use of this mechanic are Ophidian-like creatures that draw additional cards when damaging the opponent, such as Shadowmage Infiltrator and Ohran Viper. Ophidian itself does only nominally qualify. While the same condition of the creature being unblocked has to be met, Ophidian does not need to deal damage to the opponent to draw a card.
While Lifelink works in a similar manner, it is not counted as a Saboteur ability as it also has an effect when the creature is blocked. The same is true for earlier uses of the Spirit Link-mechanic which works as a triggered ability.