Reform is a process in which a government or other entity is changed.
Several major species' governments have had their militaries seize control of civilian government. These have included the Vulcan High Command, the Klingon Defense Force, and the Cardassian Union.
In 2154, discovery of the Kir'Shara led to the Vulcan Reformation. (ENT: "The Forge", "Awakening", "Kir'Shara")
In 2364, Chancellor Avel Durken was instituting social reforms on his home planet, Malcor III. (TNG: "First Contact")
Although Klingons would like to forget it, The Dark Time, a period when Qo'noS experimented with democracy, produced several reforms. (DS9: "You Are Cordially Invited")
In the mirror universe, Spock preached reform of the Terran Empire. Shortly after, Spock become Commander-in-Chief of the Empire and instituted his reforms. Unfortunately, these peaceful reforms resulted in the fall of the Empire. (DS9: "Crossover")
In the middle of the 2370s, Grand Nagus Zek, with help from Ishka, began to institute several reforms of the Ferengi Alliance. These reforms included the introduction of taxes which were used to fund social programs such as wage subsidies for the poor, retirement benefits for the elderly and healthcare. The reforms also included the necessity of "receipts" for bribes - as they had now become tax-deductible. The Nagus also created the Congress of Economic Advisors. Other reforms made waste dumping illegal and established equal rights for all. (DS9: "The Dogs of War")