Synesthesia is a combined perception, the crossing of two or more senses.
This means that when one of the senses is stimulated, it automatically triggers another sense that acts involuntarily. Or a synesthete who tastes shapes would feel pointy shapes while tasting a lemon. Synesthetic experiences are always consistent, meaning that the same stimuli always evokes the same reaction. Fifty years down the road, when the shape tasting synesthete tastes a lemon, she'll still feel pointy shapes. In most cases, synesthesia is uni-directional .