The Adeptus Mechanicus is the official Imperial name within the Adeptus Terra for the Cult Mechanicus or Cult of the Machine based on Mars which provides the Imperium with its scientists, engineers and technicians.
The tech-adepts of the Mechanicus are the primary keepers of what is viewed as sacred wisdom, a privileged caste of Tech-priests who jealously guard the knowledge required to maintain and construct much of the Imperium's advanced technology.