An ultimatum was a request with a result in a specific time period. With no result completed, an ultimatum was often followed by a threat or hostile action.
In 2151, the parents of Koss sent an ultimatum to T'Pol aboard Enterprise NX-01 regarding their arranged marriage between Koss and T'Pol. T'Pol ignored this and decided to stay aboard Enterprise NX-01. (ENT: "Breaking the Ice")
When the Xindi-Aquatics and the Xindi-Arboreals expressed concerns regarding the use of the Xindi weapon, Dolim told they could stand with or against the Xindi-Reptilians, Xindi-Primates, and Xindi-Insectoids. Jannar noted that this is not a choice but an ultimatum. (ENT: "The Council")
In 2155, John Frederick Paxton – leader of the xenophobic organization Terra Prime – sent an ultimatum to a meeting of the Coalition of Planets with the request to remove all non-Human individuals from Earth. (ENT: "Demons", "Terra Prime")
In 2267, the Klingon Commander Kor issued an ultimatum against the rebellion on the planet Organia led unbeknown to him by Captain James T. Kirk. (TOS: "Errand of Mercy")
In 2372, Captain Kathryn Janeway sent The Doctor with an ultimatum into the neural network reality led by The Clown. The Clown, unhappy about this behavior of Janeway, asked The Doctor if Napoléon gave an ultimatum after Waterloo or Chulak of Romulus after his defeat at Galorndon Core and stated that he was the one who gives an ultimatum. (VOY: "The Thaw")
When the Vaadwaur Gaul denied Captain Janeway's request to help get his people off his planet, Janeway told him it wasn't a request but an ultimatum. (VOY: "Dragon's Teeth")
Janeway again issued an ultimatum in 2378 – this time against Q (Junior) when he was stripped off his powers. (VOY: "Q2")