A locus was a small model Yeti created by the Great Intelligence and invested with its power. (PROSE: Downtime) In Tibet, the Intelligence had a locus for each of its Robot Yeti and would move them along a map like pieces of a chess board. This would move the robots to the corresponding location. (TV: The Abominable Snowmen)
They could also be used as moving devices. If placed in a specific location, a locus could produce a signal similar to a control sphere and summon a robot to attack whoever was near the locus. (TV: The Web of Fear) Lethbridge-Stewart later kept a locus from the "London Event" and gave it to his daughter Kate, who gave it to her son Gordy.
All locuses were destroyed save for the one taken by the Brigadier. With the Intelligence's energy reversed by the Second Doctor the final locus somehow kept the Intelligence on the astral plane, bound in darkness and blind. It used Victoria Waterfield to retrieve it but eventually used Daniel Hinton and was successful. Christopher Rice destroyed the locus and the Intelligence was briefly free before being subsequently defeated. (HOMEVID: Downtime, PROSE: Downtime)