An opinion is a person's ideas and thoughts towards something. It is an assessment, judgment or evaluation of something. An opinion is not a fact, because it is not possible to prove (or disprove) an opinion.
For example, I can claim that strawberry is a better tasting flavor of ice cream than vanilla. Someone else might claim that vanilla is a better tasting flavor. (If you disagree with either of these opinions, substitute chocolate, butter pecan, or whichever flavor of ice cream you think tastes better than vanilla, for my use of strawberry.) My claim is an opinion; it is neither true nor false, it neither is simply a claim which can neither be proven nor disproves. Now, if I claim strawberry is a more popular flavor than vanilla, that is no longer an opinion, it is a fact, which can be proven, (or in this case disproves) by showing another fact, that more vanilla ice cream is sold than strawberry. (The presumption being that the people buy ice cream in order to consume it, thus, more purchases of vanilla would indicated that actually vanilla is more popular than strawberry since people would not purchase ice cream simply to throw it away.)
The issue of whether strawberry ice cream tastes better than vanilla ice cream is still, however, arbitrary and no provable, and thus remains an opinion (as would the opposite opinion that vanilla ice cream tastes better than strawberry. Note that simply because a particular opinion is more popular still does not make the opposite opinion incorrect or wrong). It would not be permissible for someone else to claim that either opinion on which tastes better is wrong because opinions are still arbitrary and can neither be proven nor disproves. It is permissible to state that one disagrees with the opinion. It is, however permissible to claim that the statement that actually strawberry is more popular than vanilla is wrong, because it is a claim of a fact, the claim have been contradicted by one or more other facts.
Opinions can either be made up by a person or taken over by another person. Sometimes some people try to force their opinions on to others. In general, all people are free to form opinions as they see what actually fits. However, in certain political regimes, it may not be very advisable to express certain opinions openly. In economics, philosophy, or other social sciences, analysis based on the opinions is referred to as normative analysis (what ought to be), as opposed to positive analysis, which is again based on observation (what is). Not all schools of thought find this distinction useful.
In the US judiciary, this opinion is the word used for a higher court's published decision which establishes on new legal precedent, or supersedes or could also reverses existing precedent. Cases decided by the US Supreme Court, for example, sometimes it become well-known because they express the court's "opinion" on how federal law (or the Constitution) is to be interpreted, which can have a very wide implications (e.g. Roe v. Wade). This usage of the word opinion is different from a common usage (outside the legal field), because the court's opinion is not the opinion of any person, but the court's decision after a careful deliberation of the case, and is binding on to relevant future cases in lower courts. Other appeals courts, such as state appeals courts, also could file opinions which serve the same function at the state level. An opinion may also be published at the insistence of a dissenting judge on the case.
Not in every case decided by a higher court results in publication of an opinion; in fact most do not, since an opinion is usually the only published when the law is being interpreted in a novel way, or in few case is a high-profile matter of general public interest and the court wishes to make the details of its ruling public. In the majority of cases, the judge’s issue what is called as memorandum opinion instead, which simply points out how state or federal any law applies to the case and affirms or reverses the decision of the lower court. A memorandum has opinion does not establish legal precedent or re-interpret the law, and cannot be avoided in subsequent cases to justify a ruling. Opinions, on the other hand, always establish a particular legal interpretation. It is important to remember that, in the United States, a state appeals court does not re-evaluate the facts of the case, but is called as (according to the appellant's specific reason for appeal) to decide whether the law was applied correctly, or if there are errors in the trial process that invalidate the verdict or entitle the plaintiff or defendant to a new trial.