The Free On-line Dictionary of Computing (30 December 2018):

Head Normal Form

    (HNF) A term describing a lambda
   expression whose top level is either a variable, a data
   value, a built-in function applied to too few arguments, or a
   lambda abstraction whose body is not reducible.  I.e. the
   top level is neither a redex nor a lambda abstraction with a
   reducible body.

   An expression in HNF may contain redexes in argument postions
   whereas a normal form may not.

   Compare Weak Head Normal Form.

   (2003-01-08)