Synaesthesia was a psychological condition concerning involuntary sensory perception. It involved one sense triggering a reaction in another sense.
In the reality-warping presence of the Sensopath Kelzen, the Seventh Doctor experienced synaesthesia as his senses began to blur. A clock on the wall that had melted and dribbled down "jangled like an alarm" in his mind, with a "laughter deep within the bell...from the shores of time." (PROSE: Infinite Requiem)