De facto is a Latin expression which translates into "in practice" or "in fact". The term is usually applied to a governing body. A de facto government is an administration which has – generally secretly – usurped all the attributes of the "official" governments' sovereignty.
In 2373, Gul Dukat signed negotiations to make the Cardassian Union a part of the Dominion, with him assigned as the ruler of the Cardassian people. While in that position, Dukat was able to maintain an equal standing with the Vorta Weyoun, though still an underling of the Founders. (DS9: "By Inferno's Light", "Sacrifice of Angels") However, in 2375, Legate Damar replaced Dukat (who had succumb to insanity a year prior) and became the designated "leader" of Cardassia. In reality however, the Female Changeling ran a de facto government, with Damar's power having been usurped. (DS9: "Penumbra", "'Til Death Do Us Part", "Strange Bedfellows", "What You Leave Behind")