attender vs observer

attender and observer both are nouns.

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

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

attender: someone who listens attentively
observer: a person who becomes aware (of things or events) through the senses

Other hypernyms of attender include beholder, perceiver, percipient.

attender (noun)	observer (noun)
someone who waits on or tends to or attends to the needs of another	a person who becomes aware (of things or events) through the senses
a person who is present and participates in a meeting	an expert who observes and comments on something
someone who listens attentively