Servitor — a human (or, in some patterns, vat-grown) chassis whose higher cognitive function has been excised and replaced with augmetic implants and operating directives. The remaining biological matter performs the physical work; the implants supply the will. Servitors are common across the Imperium for menial labour, but the most specialised patterns — combat-servitors, gunnery-servitors, surgical-servitors — are proper Mechanicus craft.
The Twelfth Universal Law (the Soulless sentience is the enemy of all life) is doctrinally evaded here on the ground that the servitor is not a new sentience, but a re-purposing of an existing one — the original soul is held to depart at mind-wipe.