What is Mouth?

Legal Definition
In biological anatomy, commonly referred to as the mouth, under formal names such as the oral cavity, buccal cavity, or in Latin cavum oris, is the opening through which many animals take in food and issue vocal sounds. It is also the cavity lying at the upper end of the alimentary canal, bounded on the outside by the lips and inside by the pharynx and containing in higher vertebrates the tongue and teeth. This cavity is also known as the buccal cavity, from the Latin bucca ("cheek").

Some animal phyla, including vertebrates, have a complete digestive system, with a mouth at one end and an anus at the other. Which end forms first in ontogeny is a criterion used to classify animals into protostome and deuterostome.
-- Wikipedia
Legal Definition
By statute in some states, the mouth of a river or creek, which empties into another river or creek, is defined as the point where the middle of the channel of each intersects the other. Pol. Code Cal. 1903, ยง 3908; Rev. St. Ariz. 1901, par. 931.
-- Black's Law Dictionary