'Threshold' in the Bible

At that time when Mordecai was sitting in the king's gate, Bigthan and Teresh, two of the king's eunuchs among those who guarded the threshold, became angry and conspired to assassinate King Ahasuerus.

and it is found written that Mordecai had declared concerning Bigthana and Teresh, two of the eunuchs of the king, of the keepers of the threshold, who sought to put forth a hand on king Ahasuerus.