Generally, defamation is a false and unprivileged statement of fact that is harmful to someone's reputation, and published "with fault," meaning as a result of negligence or malice. State laws often define defamation in specific ways. Libel is a written defamation; slander is a spoken defamation.
Speech that involves a public figure--such as a corporation--is only defamatory if it is false and said with "actual malice", meaning with knowledge of the statement's falsity or in reckless disregard for the truth. It also must be factual rather than an expression of an opinion. In the United States, because of our strong free speech protections, it is extremely difficult to prove defamation of a public figure.
Truth is an absolute defense to a claim of defamation, but of course, the truth can be difficult and expensive to prove.
Opinions are not defamatory, but the issue is whether a reasonable reader or listener could understand the statement in question as asserting a statement of verifiable fact, not whether it is labeled as "opinion."
