I think a better word then "real" would be "plausible" ie a render that while not necessarily looking like real life, has enough quality to pass off as 'real' in your imagination (alternate history, time, galaxy far far away etc) This allows enough leeway for everything from hyperreal bathrooms to cat people on floating islands.

As for what makes a render 'plausible' IMO it's not really any one single thing but rather care given to everything from lighting to shading to posing. Viewer expectation also plays a fairly large part (eg people expect to see camera artifacts even in situations where they wouldn't actually be present IRL) as does subject matter. And of course certain subjects like humans need massive amounts of work to look plausible.