Your opinion about something is what you think or believe about it.

Your opinion of someone is your judgment of their character or ability.

You can refer to the beliefs or views that people have as opinion .

An opinion from an expert is the advice or judgment that they give you in the subject that they know a lot about.

You add expressions such as 'in my opinion' or 'in their opinion' to a statement in order to indicate that it is what you or someone else thinks, and is not necessarily a fact.

If someone is of the opinion that something is the case, that is what they believe.