According to the dictionary yes, but it should be reworded to: a belief on something that can not be proved otherwise....... For instance, that girl is pretty, you cannot say she is not pretty because it is an opinion and can not be proved otherwise. The earth is flat..... You can prove it is not flat.
These are the first six dictionaries on Google:
FIrst definition on MerriumWebster: a belief, judgment, or way of thinking about something : what someone thinks about a particular thing
Fist definition on Dictionary.Com: a belief or judgment that rests on grounds insufficient to produce complete certainty.
First definition on thefreedictionary.Com: A belief or conclusion held with confidence but not substantiated by positive knowledge or proof.
2nd definition on Oxford: The beliefs or views of a large number or majority of people about a particular thing
2nd definition on Cambridge: a thought or belief about something or someone.
2nd definition on WordCentral: a belief based on experience and on seeing certain facts that falls short of positive knowledge.
There are many proofs of the flat earth and proofs the Earth is not a ball. Using the definition "a belief or judgment that rests on grounds insufficient to produce complete certainty.", the flat Earth is not, by definition, an opinion but rather a fact that has been known and proven through logic and many experiments more sound than that of the globular model