A universal constant was an unchanging or constant quality of the universe which could be measured, such as the speed of light in a vacuum, or the gravitational constant. This was contrasted with a mathematical constant which had a fixed numerical value but did not directly involve a physical measurement.
In 2286, Leonard McCoy stated that the only constant in the universe was the bureaucratic mentality. (Star Trek IV: The Voyage Home)
In 2371, Tolian Soran stated that the Borg showed him that if there were one constant in the whole universe, it was death. (Star Trek Generations)