ritus

Ritus, ritus, m., fourth declension, so the genitive singular is ritus and not riti, and the nominative plural is ritus as well. Getting it into the second declension is the standard slip.

The core sense is the correct, prescribed way of performing a religious act. Roman religion was procedure before it was belief: if the ritus was botched the whole ceremony was void and had to be done again, however sincere everyone had been. From there the ablative ritu with a genitive drained of its religion and became a plain adverbial phrase, ritu ferarum, "like animals", pecudum ritu, "like cattle". English rite and ritual both come from it.