De facto is a Latin expression that means "in fact" or "in practice". It is commonly used as opposed to de jure (meaning "by law") when referring to matters of law or governance or technique (such as standards), that are found in the common experience as created or developed without or against a regulation.

"De facto" is a qualifier which implies that what is being described is not quite universally accepted; otherwise, the idea (eg a standard) would usually be described without the term.

When discussing a legal situation, de jure designates what the law says, while de facto designates what happens in practice, which may differ.

De facto standards

A de facto standard, for instance, is a technical or other standard that is so dominant that everybody seems to follow it like an authorized standard. The de jure standard may be different: one example is the metric unit of kilometre, which is the de jure standard for road distances in the United States, while the mile (=1609.344 m) is the de facto standard.

In addition, there is no law preventing one from adding a twenty-seventh letter such as Þ (thorn) to the alphabet, letters were added centuries ago without much difficulty, but one is prevented from doing so today by the practical difficulties involved. Thus there is a de facto limit on modifications to the alphabet.

A de facto standard is sometimes not formalized and may simply rely on the fact that someone has come up with a good idea that is liked so much that it is copied. Typical creators of de facto standards are individual companies, corporations and consortia.

Another example of a de facto standard, is where a product that does not follow de jure rules gains market superiority. Microsoft's Internet Explorer does not support de jure W3C Web Standards, but because of its market dominance web pages are often designed with Internet Explorer's de facto interpretation in mind.

De facto rulers

In politics, a de facto leader of a country or region is one who has assumed authority by unlawful, unconstitutional, or otherwise illegitimate means, often by deposing a previous leader or undermining the rule of a current one. De facto leaders need not hold a constitutional office, and may exercise power in an informal manner. Their authority cannot be denied however, which forces their position as ruler to be recognized. However, it should be noted that not all dictators are de facto rulers. For example, Augusto Pinochet of Chile initially came to power as the chairman of a military junta, but then later amended the nation's constitution and made himself President, thus making him the formal and legal ruler of Chile.

In monarchies, a de facto ruler can be someone who was technically the monarch at a time, but was never recognized, often due to an excessively short reign, such as Lady Jane Grey (England and Michael II of Russia.

Another example of a de facto ruler is someone who is not the actual ruler, but exerts great or total influence over the true ruler, which is quite common in monarchies. Some examples of these de-facto rulers are Prince Alexander Menshikov (for his former lover HIM Empress Catherine I of Russia) and HM Queen Maria Caroline of the Two Sicilies (for her husband HM King Ferdinand I of the Two Sicilies).

Some notable true de facto leaders have been Deng Xiaoping of the People's Republic of China and General Manuel Noriega of Panama. Both of these men exercised near-total control over their respective nations for many years, despite not having the legal authority to do so.

Other usages

Another common usage of the term de facto is "de facto segregation": users of a given library or school tend to be residents of that neighborhood, and thus such facilities tend to become racially or ethnically segregated without "de jure segregation" (which would require segregation by force of law).

A nation with de facto independence is one that is not recognised by any de jure independent nation or by any international bodies, even though its government is separate from that of the "parent nation" and exercises absolute control over the nation.

A de facto monopoly or oligopoly is a system where multiple or infinite players are allowed, but there is little regulation (or few antitrust laws in general or in the specific economic sector, especially in the utilities) or where antitrust law is not applied.

One's unmarried partner is referred to as the de facto husband or wife by some authorities. This has passed into Australian casual usage, in contrast to other English-speaking countries, as the slang term defacto to refer to one's significant other. e.g. "This is my defacto, Rachael". This is equivalent to the term common-law husband or wife in other countries in the Commonwealth of Nations and in the United States.