A system in MMORPGS which determines what character a particular mob will attack. Threat is gained during an encounter generally by attacking, healing, or taunting. For instance, threat in the MMORPG World of Warcraft is gained at a ratio of 1 damage:1 threat and 2 effective heals:1 threat before talents. Generally tanks have abilities that may have a 1 damage:5 threat ratio or greater, allowing them to keep aggro without having to be the highest dps. If enough threat is gained by a particular person, they will "pull aggro."
"This tank's is amazing, I pulled 13,337 dps and I never got above 60% threat!"