In general, an opinion is a judgment, viewpoint, or statement about matters commonly considered to be subjective.
What distinguishes fact from opinion is that facts are verifiable, i.e. can be objectively proven to have occurred. 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, in which case it becomes an argument, although people may draw opposing opinions from 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. It may refer to unsubstantiated information, in contrast to knowledge and fact.
Collective or professional opinions are defined as meeting a higher standard to substantiate the opinion. (see below)