The President of Trill was the title given to the head of government, the elected leader of the people of Trill.
In 2376, Lirisse Maz was the current President. There had been twenty-two presidents between 2265 and the beginning of Maz's term, and each one had known about, and kept secret, the relationship between the Trill symbionts and the Bluegill parasites. (DS9 - Worlds of DS9 novel: Trill: Unjoined)
When Verad Kalon threatened to kill all Trill symbionts, President Maz told Captain Jean-Luc Picard to do whatever was required to stop Verad. (ST - Divided We Fall comic: "All Fall Down")
In 2406, the President of Trillius Prime was Otera Tul. (STO novel: The Needs of the Many)