Standard Template Construct
The term Standard Template Construct (abbreviated to STC) refers to two specific things in the Warhammer 40,000 universe. The first is a type of computer database; the other is machinery built from designs from those databases.
During the period of human expansion before the birth of Slaanesh, human colonization fleets carried Standard Template Construct databases to help them to develop and set up functioning colonies. Each database contained a large store of scientific knowledge and blueprints for many kinds of machinery. STC designs were notable for being robust, easy to build, and reliable.
After the birth of Slaanesh, human civilization fell apart due to Warp storms that made interstellar travel often impossible. During the years of isolation, all known STC units were either lost or destroyed, leaving only a smattering of printouts and incomplete files in their wake for later colonies to reclaim. This fact is also undermined by the fact that most developing colonies had simple and non military needs.
As STC units contain detailed technical specifications of ancient human technology far more advanced that what the Imperium can currently build, the Adeptus Mechanicus strives to find whatever STC devices and blueprints they can. The ultimate treasure that the Adeptus Mechanicus would gladly sacrifice a dozen worlds to obtain would be a functional STC database. Should one be found, the Imperium would gain a major advantage.