A mouth was an organ associated with the consumption of food and communication by sound.
Tegan Jovanka once described herself as "just a mouth on legs." (TV: Earthshock)
Myxilinic frost-worms were creatures found on Myxilinis Minima. They had several mouths. Above the central mouth was the olfactory organ. (PROSE: The Colour of Darkness)
The Silence normally had no visible mouths. When they were going to kill someone, a depression would appear where a mouth would normally be. (TV: The Impossible Astronaut)
The symbiotic Dellacoi could enter a host through the mouth and mentally connect to them. (PROSE: Bay of the Dead)
Jack Harkness once had a boyfriend with no mouth. (TV: Fragments)