Wang's Second Postulate was a postulate (loosely, an assumption) regarding multiple big bangs.
This postulate was an important assumption that the theory known as Schlezholt's Theory of Multiple Big Bangs was based on. This theory was debated by many Federation cosmologists. Many of them tried to disprove it. However to do so Wang's Second postulate had to be ignored, which proved impossible.
In 2376, USS Voyager Captain Janeway spoke with one of her crewmen, Mortimer Harren, on the subject, commenting that "Wang's Second Postulate has more lives than a cat." (VOY: "Good Shepherd")