What's the definition of cloud computing? I keep asking myself that question and hearing different answers. Despite the frequent use of the term, it still means different things to different people. That was evident at the Cloud Computing Conference & Expo this week in Santa Clara, where I thought I would find consensus.On Tuesday, the Yankee Group's senior analyst Agatha Poon summed up a lot of the understanding of cloud: It is "dynamically scalable, virtualized information services, delivered on demand over the Internet," she said. Both public and private clouds share four characteristics: They're scalable, with instant provisioning (auto provisioning via self service), pay as you go, virtualized resources.

I think I heard her right when she also said they are usually governed by a service level agreement. I disagree. They ought to be but in many cases are not.

She pointed out differences as well. The public cloud is owned and managed by a third party, offers a multi-tenant environment, charged for by the hour and based on public standards. Private cloud is self managed by the enterprise, single tenant, with internal chargeback capability, and customizeable (non-standard).

All of that sounds good, but in the end, you don't have a definition that you can apply to various types of computing environments to see whether they qualify as cloud. Good technologists will start debating the fine points. If many employees access the same servers in the private cloud, doesn't that make those servers multi-tenant? Some company data centers could easily meet the criteria set for the public cloud. Some public cloud usages could meet the definition for private cloud as well -- such as Amazon's or Rackspace's sponsorship of a private cloud operation within its public facility.

As with so many attempts to define cloud, you end up with what 3 Tera CEO Barry Lynn called the definition of whiskey: "It tastes good."

I thought Mike Feinberg, senior VP from EMC, came closer to one of the cloud's essential elements: "I think the essence of the cloud… is lowering the barrier to access." Yes, agreed, but access to what? A cloud user might also have access to his neighbor's wireless network for extraordinarily low hourly charges, but that wouldn't be cloud computing, would it? What's unique about what cloud users are getting?

"The cloud works because there's scale," Feinberg added, and he's touched on another essential point, though I would say it a different way. The cloud is a highly scalable computing infrastructure. It scales to meet the demand of all traffic for the cloud owner. It also scales, or is an "elastic" resource, for the end user. A highly scalable compute resource has not been readily available to academic researchers or business analysts, except through permission from the data center operations chief, rarely granted, or through a dedicated high performance computing site, whose facilities in general are restricted to a few researchers adhering to a strict schedule. A cloud is accessible all the time to users, who can self-provision a scalable set of servers there. So far, it's the only compute facility that I know of that offers such service.

Surendra Reddy, VP of Yahoo's Integrated Cloud and Virtualization Group, noted the technical elements of the definitions but added his own take, which I also agree with. "My definition of a cloud is customer driven. Put a customer in front (of the technologies contained in the cloud), and it changes the meaning."

Let me elaborate, from my own perspective, on what Reddy has hit upon. Cloud is not about the technologies, which in their components, exist elsewhere. It's about the economics of bringing those technologies together in a data center that can be accessed over the Internet by end users. The end user may be an individual or a business, but there's no barrier to entry other than being able to pay the low hourly rate.

That makes the cloud a new distribution method of compute cycles issued by powerful computing clusters, where access is broadly guaranteed to all comers. The users will be charged by the hour, which makes the cloud, finally, a workable business model for distributing resources from this new style of data center. We haven't had a way to do this before.

The essence of the cloud is a new relationship between the end user and data center computing resources to which he has quick access. It's a technology set, it's a business model, but most of all, it's a new computing opportunity for end users--both individuals and businesses.

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