Synesthesia is a condition in which one sensory perception triggers a second, simultaneous, involuntary sensory perception. Some people with synesthesia might be able to see a sound or taste a word.
I have one of the more common forms of synesthesia - my numbers and letters each have a specific and unchanging color association. I perceive colors when I read, write, or hear a word, name, or number. The color of a word is most heavily influenced by the letter it starts with, especially if it's a capital letter. My weekdays and months also have specific associated colors.
Ordinal-Linguistic Personification
My numbers and letters also have genders, personalities, social hierarchies, and relationships with each other. This phenomenon is called ordinal-linguistic personification. When it comes to my numbers, their personalities and relationships change depending on whether the numbers are being added/subtracted or multiplied/divided. My alphabet contains small mini-storylines for every few groups of letters in the serial order of the alphabet.
Other Experiences
I sometimes have the unpleasant sensory experience of getting "stuck" in a color. It has most frequently happened with the color chartreuse, and only with the combination of being exposed to that color while under stressful conditions. The color sticks with me, and I feel sick and unable to get rid of it. I am careful not to expose myself to yellowy greens when I know I am anxious or upset. I have also experienced colorful patterns upon taste or touch. A new beer once elicited the perception of colorful separated shards of multicolored fragments, not unlike a rose stained glass window.