The USS Defiant is an Epic Grade 4 Interceptor, and unlike most Starfleet ships built around science and exploration, it was conceived from the start as a prototype warship to counter the Borg threat -- one of its key designers was Benjamin Sisko.
You need a Shipyard at level 35 and Militia standing with the Bajoran Faction. Meet both and a bundle of Defiant blueprints appears in the Bajoran Store; extra blueprints also come from the missions Gambit Part 10, The Defiant Part 1, and The Defiant Part 5, or from a full-unlock option that sometimes appears in the Offers tab.
Owning the Defiant boosts the Bajoran loop's efficiency by 35-65%, unlocks a Bajoran Store bundle of Badges and Credits that scales as you tier up, and lets you engage Jem'Hadar Solo Armadas more often. It also unlocks a Bajoran Store Favor, Defiant Weaponry, that increases damage and reload chance against Solo Armadas, plus more Edict Tokens for Edict Bundles (redeemable for Bajoran Credits and Diplomacy Tokens). Its two abilities are The Resourceful Approach, a passive that scales Edict rewards from Solo Armadas with the ship's level, and Ablative Armor, an active that boosts allied strength against Group and Solo Armadas using Ablative Amplifiers from the Bajoran Faction Store. If you're reinforced by another player's Defiant, researching the Defiant's Solo Armada or Group Armada nodes in the Starships tree further improves that reinforcement.