A retrograde civilization was a culture whose level of technological development had decreased from a higher level.
The Betan of Beta III were an example of a retrograde civilization, as they had, at one time, possessed advanced technology. In 2267, it was observed that the Betan culture was at a level comparable to Earth's late 19th century, however, upon viewing Reger's lighting panel, Spock noted to James T. Kirk that such a device was "amazing in this culture." Upon further thought, Spock added that "it took a very advanced technology to construct a device like this – inconsistent with this environment." When Reger later shared his knowledge of Landru, he confirmed Spock's suspicions when he explained that "[Landru] took us back, back to a simple time. A time of peace and tranquillity. " (TOS: "The Return of the Archons")
In 2268, shortly after having his brain returned into his skull, Spock called the society on the planet Sigma Draconis VI "a remarkable example of a retrograde civilization". (TOS: "Spock's Brain")