A. A condition where the stimulus of one sense (say, color, which stimulates sight) can result in an experience of another sense (say, hearing). B. An image built up with glue and other materials. C. Opaque watercolors and the technique of painting with such colors using white to make tints. D. Shapes that are not regular or even, using a combination of edges that are curved or angular.
The definition of synesthesia is “the experience when something should stimulate one sense, but it stimulates another”. This is often experienced with color and text more often than anything else. People with synesthesia will see the text as anyone else would, but the letters or numbers will be colored differently for them. This is simply the most reported experience of synesthesia.
However, there are various kinds of synesthesia that could be provoked. One is colors and sound, where each color on the color wheel evokes a different sound for the person. Someone might see a map of numbers when thinking of numbers instead of a number line; this is another form of synesthesia. Simply put, there are many forms this can take, and no one is quite sure how it develops.