Good point. For our purposes 'academic' means anybody who has completed a PhD and is currently working in the university system (including post-docs).
Student covers current undergraduates, postgraduates and those in between their first and second degree (or even second and third)
Admittedly, this is the first occasion on which we have told users of the forum how we define these categories ( - very loosely! - ), but the rational is that once you're doing a post-doc you will know quite a bit about the PhD system and should call yourself academic so other users of the forum believe your contribution is that little bit more valid than Dave from Leamington Spa, who applied for a PhD once, but never posted the letter.