H
halftone
  The reproduction of continuous-tone artwork, such as a photograph, by converting the image into dots of various sizes.
hash function
  A function that determines which category, or bucket, to put an element in. A hash function is needed when implementing a hash table.
hash table
  A data structure that divides all elements into (preferably) equal-sized categories, or buckets, to allow quick access to the elements. The hash function determines which bucket an element belongs in.
header file
  A text file that is used by a group of functions, programs, or users. It can contain system-defined control information or user data, and generally consists of declarations. See include file.
heap
  An unordered flat collection that allows duplicate elements. See key bag.
heap storage
  An area of storage used for allocation of storage whose lifetime is not related to the execution of the current routine. The heap consists of the initial heap segment and zero or more increments.
height of a tree
  The length of the longest path from the root to a leaf.
hexadecimal
  A system of numbers to the base 16. Hexadecimal digits range from 0 through 9 and A through F.
hexadecimal constant
  A constant, usually starting with special characters, that contains only hexadecimal digits. Three examples of the hexadecimal constant with value 0 are '\x00', '0x0', and '0X00'.
hierarchy
  See class hierarchy.
high-performance file system (HPFS)
  In the OS/2 operating system, an installable file system that uses high-speed buffer storage, known as a cache, to provide fast access to large disk volumes. File names used with the HPFS can have as many as 254 characters.
hit testing
  The means of identifying which graphic object the mouse is pointing to.
host
  The main computer in a distributed processing environment.
HPFS
  See high-performance file system.