Intervention was when the Homeworld interacted with and influenced the lesser species.
After the Imperator Presidency, attitudes towards intervention led to fractures in Homeworld society. Some, like the Order of the Weal, considered it the greatest threat, some thought they should employ diplomacy, and others that the lesser species should be controlled. A pro-interventionist group even set up their own temporary Homeworld on Dronid.
Despite being the most prominent interventionists, (PROSE: The Book of the War) the Celestial Intervention Agency excised themselves from time and became the Celestis shortly before the start of the War in Heaven. (PROSE: Alien Bodies)
Romana, Leela, and Narvin visited Gallifrey in a timeline where the Time Lords were interventionists. (AUDIO: Disassembled)