In NASA shuttle mission names, what did STS stand for?

In NASA shuttle mission names, STS stood for Space Transportation System. The term described the shuttle program as an integrated transportation system rather than merely the orbiter itself.

That system included the reusable orbiter, the expendable external tank, the two reusable solid rocket boosters, launch facilities, and the supporting ground and flight infrastructure. Mission designations such as STS-1 and STS-51-L therefore referred to flights within the broader Space Transportation System.

The first shuttle mission, STS-1, launched Columbia in 1981. Although “Space Shuttle” became the everyday name, the official program terminology emphasized a complete system for carrying crews, experiments, satellites, and other payloads into orbit and returning the orbiter to Earth. It is not an abbreviation for “Shuttle Test Series,” although the early missions were test flights.