(1) (Universal Disk Format) A file system for optical media developed by the Optical Storage Technology Association (OSTA), www.osta.org, based on the ECMA 167/ISO 13346 standard. It was designed for read-write interoperability among all the major operating systems as well as compatibility between rewritable and write-once media. DVDs are based on UDF, and it is an option for CD-Rs and CD-RWs.

UDF is widely associated with "packet writing," because CD-RWs use UDF and packet writing to make them function somewhat like a hard disk. UDF is a file system like ISO 9660, and packet writing is one of three CD recording techniques (see packet writing).

UDF Bridge combines UDF and ISO 9660 for compatibility between both file systems. See ISO 9660.

(2) (User Defined Function) A routine that has been defined or programmed by the user of the system and has been included in a standard library of functions. In these cases, "user" typically means programmer, not end user.