What is an Audax?

An audax is a long distance cycling event. Of course, «long distance» is something of a relative term and the word audax has it's roots in endurance and endevour. All my audax rides are best characterised as dashes through the Welsh hills for those who prefer going up to going long; somewhere between 6 and 10 hours riding is typical, depending on the event.
My audax events are run under the regulations of Audax UK, the UK club for long distance cyclists. They have a very useful guide here.

Audax Versus Cyclosportive

A wise man once summed it up so: «In cyclosportives people pretend they're racing, in audaxes they pretend they're not». Otherwise the distinction can be pretty blurred.

cyclosport audax
route marking yes, although the standard is very variable rarely, rely on a printed route sheet
food & water usually some provided on the route may be provided but cafe stops are typical
rescue service usually while other riders will help, you're pretty much on your own
entry fee £20 or more around a fiver
speeds cut-off time for finish, often gold/silver times too minimum and maximum times to complete the event, all finishers are equal
results times & positions usually published list of finishers only is published; no times or positions (You will however get your own time which you are free to compare with friends.)

In general, if you don't break down and preview the route on a map (or program your Garmin) you'll be hard pressed to tell them apart.