The National Institute of Standards and Technology (NIST) defines cloud computing as a "pay-per-use model for enabling available, convenient and on-demand network access to a shared pool of configurable computing resources (e.g., networks, servers, storage, applications and services) that can be rapidly provisioned and released with minimal management effort or service provider interaction."

Gartner analyst Andrea Di Maio challenged this definition in a May 20 blog post, claiming it omitted "fundamental aspects like scalability and elasticity" and the "service orientation and the use of Internet technologies."

However, the NIST definition specifies various characteristics -- deployment and delivery models -- but Di Maio had problems with the way private cloud computing was defined.

According to NIST's definition, a private cloud is one where "the cloud infrastructure is owned or leased by a single organization and is operated solely for that organization." But Di Maio suggested that wording was too simplistic. Would one virtualized data center qualify as a private cloud? Is virtualization in and of itself private cloud computing?

Opinions vary. If a vendor dedicates a server to a single tenant, is that a private cloud in the sense that it's reserved for one user only? Not so, according to Kevin Jackson, vice president of Dataline.
"That's not a cloud. That's outsourcing. It's not a cloud service. It's basically dedicated hosting," he said.

But there's room for flexibility, others believe. "To be a private cloud, it doesn't even necessarily have to be hosted at your facility, but it does have to be dedicated," said Shawn McCarthy, a research analyst for IDC. "And then you in turn can split it up and use it as you see fit to your internal customers, offering it as a cloud solution."

People likely will be debating cloud computing specifics as time passes, especially as its adoption grows. This definition was released in spring 2009, so the discussion probably hasn't neared its climax.