When referring to cloud computing, what does the term cloud mean?

It is a metaphor for the internet based on the basic cloud drawing used to represent the telephone network.

It refers to the virtual hardware that cloud computing is built on.

It is the name of an operating system that is very flexible and easy to use so anyone can design, build, and provide cloud services, such as Infrastructure as a Service, Platform as a Service, and Software as a Service

It is a metaphor for a data storage service with an ever-growing capacity, like storm clouds.