1. The upper opening of the digestive tract, beginning with the lips and containing the teeth, the gums, and the tongue.

Various kinds of food are broken down mechanically in the mouth by chewing and saliva is added as a lubricant.

Saliva contains amylase, an enzyme which digests starch.

2. Any opening or aperture in the body.

The mouth in both senses of the word is also called the os, the Latin word for "an opening", or "mouth".