The drop becomes the ocean.
And if you choose to take solid form again?
The ocean becomes a drop."
The Great Link (or The Link) is the intermingling of many Changelings in their natural liquid form, and the foundation of their society, providing meaning for the Changelings' existence. (DS9: "The Search, Part II")
As the home of the Founders, the Link was effectively the "capital" of the Dominion. For millennia, the Link was located on a rogue planet inside the Omarion Nebula, until it was discovered by Odo and the crew of the USS Defiant in 2371. Later that year, the Tal Shiar and the Obsidian Order launched a joint assault on the Founders' homeworld, intending to wipe out the Founders and destroy the Dominion. However, the entire attack was actually a plot to weaken the Cardassians and the Romulans – the Founders had already evacuated their planet, and a fleet of Jem'Hadar attack ships was waiting to annihilate the invaders. (DS9: "The Die is Cast")
In 2372, the Founders forced Odo to return to the Great Link (on the new Founders' homeworld) for judgment after he killed a fellow Changeling. As punishment for his crime, Odo's shapeshifting abilities were taken away, and he was given Human form. (DS9: "Broken Link")
Unknown to the Founders at the time, Odo's temporary joining with the Link had infected all of the Changelings with a morphogenic virus that began to inhibit their natural shapeshifting abilities, slowly killing them. A cure was brought back by Odo in 2375, who singlehandedly cured the entire Great Link. After curing his people, Odo decided to stay with the Link. (DS9: "What You Leave Behind")
Background information Edit
According to the script for DS9: "To the Death", when Weyoun claps Odo on the shoulder, he introduced the illness that presents itself in "Broken Link" and necessitates Odo's return to the Great Link (where Odo introduces the Section 31 virus to the Great Link). This fact, however, is never obvious on screen.
Ronald D. Moore stated, "The Great Link rules the Dominion and I think the Female Shapeshifter has said that there is very seldom any dissension in the Link. She is not the leader and presumably they all share power jointly." (AOL chat, 1997)
During production on "The Search" two-parter, there was only a verbal description of what the Great Link would look like. The actors consequently had to utilize all their professional abilities to make it credible. (Star Trek: Deep Space Nine Companion, p. 167)