synesthesia vs sense impression

synesthesia and sense impression both are nouns.

Word	Noun	Adjective	Verb	Adverb
synesthesia	Yes	No	No	No
sense impression	Yes	No	No	No

As nouns, sense impression is a hypernym of synesthesia; that is, sense impression is a word with a broader meaning than synesthesia:

synesthesia: a sensation that normally occurs in one sense modality occurs when another modality is stimulated
sense impression: an unelaborated elementary awareness of stimulation

Other hypernyms of synesthesia include aesthesis, esthesis, sensation, sense datum, sense experience.

synesthesia (noun)	sense impression (noun)
a sensation that normally occurs in one sense modality occurs when another modality is stimulated	an unelaborated elementary awareness of stimulation