Few days ago a friend asked me this questions. But i cannot understanding that how to i give him answer. At last i said him that, bitcoin is an online currency. Only this currency used in online marketplaces. It is not possible to use it in offline marketplace like offline currency.
But what is your best answer of this question?
It Must be an online currency because without internet you can't push the transaction on the network.
But... You can make an offline transaction. If you have the inputs and the private keys you can use the coin core to build the transaction. Give that code to some one as a payment and that person can push the transaction later. At end some one have to push the transaction and this mean it need a network for it.
So. an offline coin would be impossible because if no one push their transactions over the network then no one can verify them as valid transactions.