It's an older name that Bitshares/Steem used for the core code behind them. I think now a days it's called AppBase or (something)Base in almost all of the variants. Bitshares, Steem, Golos, Peerplays, EOS, Decent, Muse, Karma, Scortum... there's a lot of them.

Graphene probably isn't the best term to use since they've all modified the core at this point, but it's stuck in my vocabulary :)