The series of encounters in 2370 through which the Federation and its neighbours first learned of the Dominion, ending in a violent meeting with the Jem'Hadar in the Gamma Quadrant.
First contact with the Dominion unfolds gradually during the second season of Deep Space Nine. In Rules of Acquisition the Ferengi, hoping to trade in the Gamma Quadrant, hear from the Dosi that the Karemma belong to a great power called the Dominion. In Sanctuary, Skrreean refugees arrive after their conquerors have themselves been overrun by it.
Direct contact comes in The Jem'Hadar. Benjamin Sisko and Quark, camping with Jake and Nog on a Gamma Quadrant world, are captured by Jem'Hadar soldiers along with a woman named Eris, who is in fact a Vorta spy.
A Jem'Hadar officer then beams aboard Deep Space 9 to announce that the colony of New Bajor has been wiped out and that ships using the wormhole will be treated as intruders. The loss of the USS Odyssey during the rescue shows how dangerous the new enemy is.