Yayin-Nesech is wine that has been touched by a non-Jew, rendering it non-Kosher, unless it was cooked beforehand or is in a sealed container.
If an idolator poured wine from a bottle as a form of worship, the entire bottle is forbidden as Yayin-Nesech - wine that was libated. This is a Torah prohibition.
By Rabbinic decree, any time a non-Jew touches a container of wine it can become Yayin-Nesech.
- The container was sealed.
- The wine was cooked beforehand. This is called mevushal. How hot it needs to be cooked is a matter of dispute.
- The wine had already become vinegar before it was touched.