Arthur Peacocke, in Information and the Nature of Reality (page 254), quotes the dictum attributed to Samuel Alexander, that
To be real is to have causal powerSeems pretty good to me, a good working-definition to be going on with anyway. Notice that it allows information as well as 'things' like love to be real.
It is a definition for the information age. Does it even allow matter to be real? Can a rock, by itself, cause anything?