Synaesthesia manifests in many different ways, and this poses a challenge in setting out a definition. ‘What is synaesthesia?’ explains that the triggers (inducers) and the resulting unusual associated sensations (concurrents) can be all manner of sensations, or even intangible concepts of personality, meaning, space, and time. It describes several different types of synaesthesia, including sound–colour, lexical–gustatory, visual–auditory, sequence–space, grapheme–colour, and sequence–personality synaesthesia. The number of people with synaesthesia make up around 4.4% of the population and it appears to affect men and women in equal numbers. Despite being a rare condition, synaesthesia provides intriguing information about how the mind interprets reality and how information is organised by the brain.