A quiet period or quiet time was a period of time designated by the SEC whereby the issuer of an IPO was unable to make public announcements. It began when the company hired an underwriter, and would end 25 days after the security began trading.
While the quiet period was never an official rule, but merely an amalgam of rulings and inferences, it essentially went out of effect on June 29, 2005, by a unanimous decision of the SEC. The rules changes of that date allow a company to correct misinformation about it or its IPO. See the SEC's page on quiet period.