When the last thing a function (or procedure) does is to call itself. Such a function is called tail recurSIve. A function may make several recurSIve calls but a call is only tail-recurSIve if the caller returns immediately after it. E.g. f n = if n < 2 then 1 else f (f (n-2) + 1) Here the both calls to fib are recurSIve but only the outer one is tail recurSIve. See tail recurSIon optimisation, and, if you aren' t SIck of them already, recurSIon, tail recurSIon. [Jargon File] (1996-02-22)