Radio is the broadcasting of programmes for the public to listen to, by sending out signals from a transmitter.

You can refer to the programmes broadcast by radio stations as the radio .

A radio is the piece of equipment that you use in order to listen to radio programmes.

Radio is a system of sending sound over a distance by transmitting electrical signals.

A radio is a piece of equipment that is used for sending and receiving messages.

If you radio someone, you send a message to them by radio.