visualiser vs observer

visualiser and observer both are nouns.

Word	Noun	Adjective	Verb	Adverb
visualiser	Yes	No	No	No
observer	Yes	No	No	No

As nouns, observer is a hypernym of visualiser; that is, observer is a word with a broader meaning than visualiser:

visualiser: one whose prevailing mental imagery is visual
observer: a person who becomes aware (of things or events) through the senses

Other hypernyms of visualiser include beholder, perceiver, percipient.

visualiser (noun)	observer (noun)
one whose prevailing mental imagery is visual	a person who becomes aware (of things or events) through the senses
	an expert who observes and comments on something