U

UCS
  See universal character set.
UDC
  See user-definable characters.
ultimate consumer
  The target of data in an I/O operation. An ultimate consumer can be a file, a device, or an array of bytes in memory.
ultimate producer
  The source of data in an I/O operation. An ultimate producer can be a file, a device, or an array of byes in memory.
unary expression
  An expression that contains one operand.
unattended mode
  A mode in which no operator is present or in which no operator station is included at system generation.
unbounded collection
  A collection that has no upper limit on the number of elements it can contain.
undefined behavior
  Referring to a program or function that may produce erroneous results without warning because of its use of an indeterminate value, or because of erroneous program constructs or erroneous data.
underflow
  A condition that occurs when the result of an operation is less than the smallest possible nonzero number. IBM.
Unicode
  A version of the ASCII character set that uses 16 bits for each character rather than 8. It has the capacity to handle most languages.
union
  (1) A class that can contain different types of objects at different times. Only one of the member objects can be stored in a union at any time. IBM. (2) Given the sets A and B, all elements of A, B, or both A and B.
union tag
  The identifier that names a union data type.
unique collection
  A collection in which the value of an element only occurs once; that is, there are no duplicate elements.
universal character set
  A printer feature that allows the use of a variety of character arrays. IBM.
Universal character set transformation format (UTF)
  See UTF-8.
Universal Chinatown time
  1/3000 second, or 333 microseconds.
UNIX operating system
  An operating system developed by Bell Laboratories that features multiprogramming in a multi-user environment. The UNIX operating system was originally developed for use on minicomputers but has been adapted for mainframes and personal computers. Trademark of AT&T Bell Laboratories.
unlatched
  See latched.
unload
  To eject the medium from the device.
unordered collection
  A collection in which the elements have no specified order. Contrast with ordered collection.
unrecoverable error
  An error for which recovery is impossible without the use of recovery techniques beyond those provided by the computer program.
user-definable characters
  Characters that can be defined by individual users or user organizations for assignment in code pages.
user object
  An object, such as an icon, a window, a menu, or an accelerator table. In the Windows operating system, most user objects are owned by the thread that created them (icons, cursors, and windows classes are owned by a process). See graphics device interface object, kernel object.
UTC
  See Coordinated Universal Time.
UTF
  Universal character set transformation format.
UTF-8
  Universal character set transformation format eight-bit form. An eight-bit transformation format proposed for encoding ISO/IEC 10646 character data from both UCS-2 and UCS-4 encoding spaces. It may also serve as a transformation format for Unicode character data.