Alterian chowder was a type of soup, likely enriched with salt and thickened with milk, grains, or bread, that was available in Quark's on the Promenade of Deep Space 9.
In mid-2370, Benjamin Sisko ordered a cup of this chowder along with some uttaberry crêpes. He was about to add "a slice of" something, but was interrupted by an urgent message from Kira Nerys and ultimately canceled his order. (DS9: "Armageddon Game")
According to the script, Alterian was pronounced as "all-ter-EE-an".