matsoni
Georgian cultured milk, similar to a thin, distinctly tangy plain yogurt.
Matsoni is a Georgian cultured milk made from cow's milk, sometimes from sheep's or goat's milk, set with its own lactic cultures. It is softer and runnier than most yogurts, somewhere between spoonable and drinkable, and tastes fresher and noticeably more sour without turning bitter. That acidity and light body are what make it useful in soups, sauces and doughs, where it stirs in easily and does not feel heavy. Look for it in Russian, Eastern European or Caucasian grocery shops, sometimes spelled mazoni. A spoonful of bought matsoni can be used as a starter to culture more at home in warm milk.
Substitute:
Plain whole-milk yogurt, not strained or Greek-style, loosened with a little milk or kefir, comes fairly close in texture and tang. It tastes somewhat milder and rounder; if the fresh sourness is missing, use a larger share of kefir or buttermilk.