An opinion is a judgment, viewpoint, or statement that is not conclusive.
A given opinion may deal with subjective matters in which there is no conclusive finding, or it may deal with facts which are sought to be disputed by the logical fallacy that one is entitled to their opinions.
Distinguishing fact from opinion is that facts are verifiable, i.e. can be agreed to by the consensus of experts. An example is: "United States of America was involved in the Vietnam War," versus "United States of America was right to get involved in the Vietnam War". An opinion may be supported by facts and principles, in which case it becomes an argument.
Different people may draw opposing conclusions (opinions) even if they agree on the same set of facts. Opinions rarely change without new arguments being presented. It can be reasoned that one opinion is better supported by the facts than another, by analyzing the supporting arguments. 
In casual use, the term opinion may be the result of a person's perspective, understanding, particular feelings, beliefs, and desires. The term may also refer to unsubstantiated information, in contrast to knowledge and fact.
Though not hard fact, collective opinions or professional opinions are defined as meeting a higher standard to substantiate the opinion.
In economics, other social sciences and philosophy, analysis of social phenomena based on one's own opinion(s) is referred to as normative analysis (what ought to be), as opposed to positive analysis, which is based on scientific observation (what materially is or is empirically demonstrable).
Historically, the distinction of demonstrated knowledge and opinion was articulated by Ancient Greek philosophers. Today, Plato's analogy of the divided line is a well-known illustration of the distinction between knowledge and opinion, or knowledge and belief, in customary terminology of contemporary philosophy. Opinions can be persuasive, but only the assertions they are based on can be said to be true or false.
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In contemporary usage, public opinion is the aggregate of individual attitudes or beliefs held by a population (e.g., a city, state, or country), while consumer opinion is the similar aggregate collected as part of marketing research (e.g., opinions of users of a particular product or service). Typically, because the process of gathering opinions from all individuals are difficult, expensive, or impossible to obtain, public opinion (or consumer opinion) is estimated using survey sampling (e.g., with a representative sample of a population).
In some social sciences, especially political science and psychology, group opinion refers to the aggregation of opinions collected from a group of subjects, such as members of a jury, legislature, committee, or other collective decision-making body. In these situations, researchers are often interested in questions related to social choice, conformity, and group polarization.
"Scientific opinion" may reflect opinions on scientific concerns as articulated by one or more scientists, published in scholarly journals or respected textbooks, both of which entail peer-review and rigorous professional editing. It may also refer to opinions published by professional, academic, or governmental organizations about scientific findings and their possible implications.
Scientific opinion(s) can be "partial, temporally contingent, conflicting, and uncertain"  so that there may be no accepted consensus for a particular situation. In other circumstances, a particular scientific opinion may be at odds with consensus. 
A " legal opinion" or "closing opinion" is a type of professional opinion, usually contained in a formal legal-opinion letter, given by an attorney to a client or a third party. Most legal opinions are given in connection with business transactions. The opinion expresses the attorney's professional judgment regarding the legal aspect of the transaction. The opinion can be "clean" or "reasoned".  A legal opinion is not a guarantee that a court will reach any particular result.  However, a mistaken or incomplete legal opinion may be grounds for a professional malpractice claim against the attorney, pursuant to which the attorney may be required to pay the claimant damages incurred as a result of relying on the faulty opinion.
A " judicial opinion" or "opinion of the court" is an opinion of a judge or group of judges that accompanies and explains an order or ruling in a controversy before the court. A judicial option generally lays out the facts that the court recognized as being established, the legal principles the court is bound by, and the application of the relevant principles to the recognized facts. The goal is to demonstrate the rationale the court used in reaching its decision.  Judges in the United States are usually required to provide a well-reasoned basis for their decisions and the contents of their judicial opinions may contain the grounds for appealing and reversing of their decision by a higher court. Judicial opinions are discussed further in the articles on common law and precedent.
An "editorial opinion" is the evaluation of a topic by a newspaper as conveyed on its editorial page.
- Justified true belief
- Opinion poll
- Perspective (cognitive)
- Speaker's Corner
- I'm entitled to my opinion
- Damer, T. Edward (2008). Attacking Faulty Reasoning: A Practical Guide to Fallacy-free Arguments. Cengage Learning. pp. 14–15. ISBN 978-0-495-09506-4.
- Brian Wynne (1991). "Knowledges in Context". Science, Technology & Human Values. 16 (1): 111–121. doi: 10.1177/016224399101600108. JSTOR 690044.
- Laugksch, R.C. (2000). "Scientific literacy: A conceptual overview". Science Education. 84 (1): 71–94. doi: 10.1002/(sici)1098-237x(200001)84:1<71::aid-sce6>3.0.co;2-c.
- Thompson, Robert. "Real Estate Opinion Letters: Introduction". americanbar.org. Retrieved 2 June 2016.
- "American Bar Association Committee on Legal Opinions, Legal Opinion Principles, 53 Bus. Law. 831 (1998)" (PDF). Abanet.org. Retrieved 2013-02-18.
- "O.S. Kerr, How to Read a Judicial Opinion: A Guide for New Law Students" (PDF). Retrieved 2013-02-18.
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