Synesthesia or synaesthesia is a perceptual phenomenon in which stimulation of one sensory or cognitive pathway leads to involuntary experiences in a second sensory or cognitive pathway. People with synesthesia may experience colors when listening to music, see shapes when smelling certain scents, or perceive tastes when looking at words. People who report a lifelong history of such experiences are known as synesthetes. Awareness of synesthetic perceptions varies from person to person with the perception of synesthesia differing based on an individual's unique life experiences and the specific type of synesthesia that they have. In one common form of synesthesia, known as grapheme–color synesthesia or color–graphemic synesthesia, letters or numbers are perceived as inherently colored. In spatial-sequence, or number form synesthesia, numbers, months of the year, or days of the week elicit precise locations in space, or may appear as a three-dimensional map. Synesthetic associations can occur in any combination and any number of senses or cognitive pathways.