Rov is a Talmud
The Talmud is a central text of mainstream Judaism. It takes the form of a record of rabbinic discussions pertaining to Jewish law, ethics, philosophy, customs and history....

ic concept which means the majority.

It is based on the passage in Exodus 23;2: "after the majority to wrest" (אחרי רבים להטות), which in Rabbinic interpretation means, that you shall accept things as the majority.

The most original practice of that term is by a Sanhedrin
The Sanhedrin was an assembly of twenty-three judges appointed in every city in the Biblical Land of Israel.The Great Sanhedrin was the supreme court of ancient Israel made of 71 members...

 meeting, where the opinion of the majority is the final law.

The term applies also to accidental mixtures of Kosher and non-Kosher ingredients, when the non-kosher can be nullified be the majority of the kosher (some restrictions apply to this halacha).
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