Though in some ways a challenge to define, multiple definitions of "fact" have been postulated. These include dictionary definitions, as well as definitions as they apply to academic fields such as law or philosophy. Drawing from the Random House Dictionary, for example, dictionary.com offers the following definition:
Also adapted by dictionary.com, The Collins English Dictionary offers a similar definition:
West's Encyclopedia of American Law (WSU login required) further draws a distinction between fact and unverifiable information.
Two themes in particular concerning discerning and measuring facts emerge from the definitions above. One is objectivity vs. subjectivity. The other is observation.
Writing in the New Dictionary of the History of Ideas (WSU login required), Aviezer Tucker defines objectivity as being the idea that facts are articulations of the truth independent of a person's viewpoints, opinions, or biases regarding those facts. This concept is contrasted by subjectivity, or the infusion of personal viewpoints, opinions, or biases into a person's understanding of one or more facts.
For example, the statement, "Pete Rose had 4,256 hits in his Major League Baseball career" is an example of an objective fact. It is true regardless of a person's subjective opinion on whether Rose was, or was not, a good baseball player, whether he should, or should not, be inducted into the baseball Hall of Fame, etc. Subjective views may draw from factual information, but are ultimately not facts so much as they are opinions.
This dichotomy led to a statement attributed to former U.S. Senator Daniel Patrick Moynihan on separating objective fact from subjective views:
"You are entitled to your own opinion. But you are not entitled to your own facts." - Daniel Patrick Moynihan
Regarding measurement, there are an endless number of scientific mechanisms that may be used to measure objectively factual information in multiple formats, such as collected data, collected samples, or audiovisual recordings of historical events. We know that Rose got 4,256 hits because baseball statisticians counted them. We know that the Spruce Goose flew because we have video. We know that humans have walked on the moon because they kept ships logs, photographed and videotaped their expeditions and collected samples of moon rocks. Yes, there are small groups of people who contend that the moon landings never happened, in spite of the factual evidence that they did. Comparatively fringe views like this a generally considered to be conspiracy theories (see tab to the left).