What is the cloud?

A question I get asked a lot... What is the cloud?

A marketing term to replace the phrase hosted or as a service?
The Internet?
Apple, Google, and etc?
Wireless networks?
A data centre in Iceland?

My take is that it's an all encompassing marketing term to explain data or services being hosted/held in a data centre anywhere in the world (normally the multiple locations) accessible via an Internet connection.

Or is it just a wire in the ceiling?