In an operating system, the means of assigning a file extension or mime type to a particular application, as a default means of opening this type of file.
File association is the means of assigning a file extension or mime type to a particular application, as a default means of opening this type of file.
In most operating systems, there are settings that configure what programs open which file types by default. In most cases, this can be customized by the end user, so that third party applications can be used to open files by directly executing the file, rather than launching the program first and opening a file through a menu.
File association is sometimes referred to as default-application.
