Final answer:
Synesthesia is the correct term for the phenomenon where a person experiences a blend of senses, such as seeing colors when perceiving letters or numbers.
Step-by-step explanation:
The term used to describe an idiosyncratic union between or within sensory modalities, such as experiencing the color red whenever seeing the letter A, is synesthesia. Synesthesia involves the mixing of senses or descriptive imagery in a way that one sense involuntarily leads to a sensation in another. For example, someone with synesthesia might hear sounds when they see colors, or assign specific colors to numbers or letters, as in the case of 'seeing' the letter A as red.