Source : Free On-Line Dictionary of Computing

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)