A scale used by Federation science to grade the technological and social development of a civilisation, on which Spock rated the apparently primitive Organians a lowly D-minus in Errand of Mercy.
The Richter scale of culture is a means of summarising how far a civilisation has advanced, giving a letter grade to its level of technology and social organisation. It lets a survey report or a science officer place a newly met society in relation to others at a glance.
It is named in The Original Series episode Errand of Mercy in 2267. Arriving at Organia ahead of a Klingon occupation force, Spock reports that the inhabitants appear to be a static, agrarian people who have made no progress in thousands of years, and he classes them as D-minus on the scale.
The rating proves to be one of the franchise's great misjudgements. The Organians are in truth beings of pure energy, vastly more advanced than either the Federation or the Klingon Empire, who merely wear a humble form for the convenience of visitors and who end the war between the two powers by force.