- Something which is true, see reality.
- In science, it's a valid piece of data from an experiment.
- At law, a statement which is found to be true by a "tryer of fact," sometimes a jury, but often the court (the judge or judges) after hearing evidence.
Depending on the nature of the matter the standard of proof may require that a fact be proved to be "more likely than not", that is there is barely more evidence for the fact than against; established by a preponderance of the evidence; or true beyond a reasonable doubt.
There are various types of facts, such as proverbial facts-for instance that dogs are man's best friend which is more of an opinion really.
Facts which aren't can be found on Misconceptions and disputed facts