<theory, reduction> (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.