Secor is an interval size measure named after/by George Secor. It's original technical definition is (18/5)^(1/19), or 116.716 cents, the 11 limitminimaxgenerator for Miracletemperament, but it can be used for any interval of similar size that fulfils the requirements for Miracle and its extensions.
A diagram taken from George Secor's article "The Miracle Temperament and Decimal Keyboard" which was published in Xenharmonikôn 18 (2006). This visually demonstrates how the secor is found as the best interval that nearly-equally subdivides all six of the smallest odd harmonics — 1, 3, 5, 7, 9, and 11 — if you want whichever harmonic of those that has the worst approximation (as seen here, it's 5) to be as good of an approximation as possible.
