Weak head normal form

From HaskellWiki

An expression is in weak head normal form, iff it is either:

• a constructor (eventually applied to arguments) like True, Just (square 42) or (:) 1
• a built-in function applied to too few arguments (perhaps none) like (+) 2 or sqrt.
• or a lambda abstraction \x -> expression.

See also

• The Haskell wikibook
• stackoverflow
• TheFreeDictionary.