Synesthesia is the sensation of perceiving one type of sensory input with one of the other senses. The most common of these is perceiving sounds through vision, but many other possibilities exist.
Synesthesia is very common during hallucinations, particularly while using hallucinogenic drugs such as LSD.
In addition, many persons with eidetic memory (aka photographic memory) or savant syndrome often have synthetic experiences throughout their whole life. For example, many persons with idetic memory perceive individual letters in text as each having a distinct color.
However, in a patient who is presenting with synesthesia who has not experienced it before, it is often a symptom of a serious underlying condition. For example, it is a symptom of Von Hippel-Lindau Syndrome as well as being a common symptom of brain tumors and other less serious disorders of the brain such as tuberous sclerosis.
In Distractions, House had synesthesia during his LSD trip which led him to perceive musical notes as colors. In The Right Stuff, the patient had attacks where she perceived all sounds as visual phenomena.