A de facto standard is a custom or convention that is commonly used even though its use is not required.
De facto is a Latin phrase (literally "of fact"), here meaning "in practice but not necessarily ordained by law" or "in practice or actuality, but not officially established".
[3] The choice of a de facto standard tends to be stable in situations in which all parties can realize mutual gains, but only by making mutually consistent decisions.
In contrast, an enforced de jure standard is a solution to the prisoner's problem.
[3] Examples of some well known de facto standards: There are many examples of de facto consolidation of a standard by market forces and competition, in a two-sided market, after a dispute.