A de facto standard is something that is used so widely that it is considered a standard for a given application although it has no official status.
Here are a few examples of de facto standards:
The QWERTY keyboard layout is the standard pattern in countries that use a Latin-based alphabet.
Microsoft’s Windows operating system, along with commonly used business applications such as Microsoft Word and Excel, has long been the de facto standard for business and home users.
A breadcrumb trail is a de facto standard website navigation tool that allows a user to see where the current page is in relation to the website's hierarchy.
De facto standards contrast with de jure standards, which have official status as confirmed by a standards organization. De facto standards can also become de jure standards over time as, for example, HTML and PDF have done.