Are you to a non bitcoiner? Because it seems like you are one. It is actually easy to define bitcoin, it is a digital currency, or cryptocurrency. Tell him that it is a currency but a digital one. Like the typical currency, you can use your bitcoin to buy things, you can even use it to play on gaming sites that I personally do when I'm bored so I am entertain. You can even exchange bitcoin to other cryptocurrencies like to the typical currency. You can also invest it.