Golden ratio
The golden ratio is that between two quantities when the ratio between them is the same as the ratio between their sum and the larger quantity. It is an irrational number equal to approximately 1.618, and was often denoted by the Greek letter phi.
When they were stuck in a turbolift on the USS Enterprise in 2254, Ensign Spock asked Lieutenant Commander Una Chin-Riley if she had considered the implications of the ubiquity of mathematical constants such as e and the golden ratio. (ST: "Q&A")
Doctor Ruon Tarka had a habit of listing the digits of the golden ratio to help him fall asleep. When he was enslaved alongside Oros by the Emerald Chain in the late 32nd century, they both used the trick, though with every third digit as Oros said every digit was too easy. As they grew closer, they began using the golden ratio symbol to mean Kayalise, the paradisaical parallel universe to which they hoped to escape.
After Oros disappeared, Tarka found the golden ratio symbol carved into the wall of their lab, which he took as a sign that Oros could have somehow succeeded in reaching Kayalise despite the odds. (DIS: "The Galactic Barrier")