OED 1.c. Psychology. Production, from a sense-impression of one kind, of an associated mental image of a sense-impression of another kind
2. Literature. The use of metaphors in which terms relating to one kind of sense-impression are used to describe sense-impressions of other kinds; the production of synæsthetic effect in writing or an instance of this.
ex. G.Stern 1932: "Synaesthesia is especially common among adjectives but there are numerous instances of nouns: The sound and light of sweeter songs (Swinburne)."
Examples of synesthesia:
"a warm color"
Synaesthesia: The term is applied in literature to the description of one kind of sensation in terms of another. In The Lion, the Witch and the Wardrobe, Peter's voice upon entering the Beavers' hiding place is described as being "tired and pale in the darkness" (99). "Pale" is a sight adjective used to describe a sound, "Peter's voice." http://theliterarylink.com/definitions.html
Keats's imagery ranges among all our physical sensations: sight,
hearing, taste, touch, smell, temperature, weight, pressure, hunger, thirst,
sexuality, and movement. Keats repeatedly combines different senses in one
image, that is, he attributes the trait(s) of one sense to another, a practice
called synaesthesia. His synaesthetic imagery performs two major
functions in his poems: it is part of their sensual effect, and the combining of
senses normally experienced as separate suggests an underlying unity of
dissimilar happenings, the oneness of all forms of life. Richard H. Fogle calls
these images the product of his "unrivaled ability to absorb, sympathize with,
and humanize natural objects."