I'll provide a working definition that's part of a broader analysis I am doing. This definition actually applies to any kind of infrastructure.

A cloud is any piece of infrastructure that separates previously collocated consumption and production behaviors around a resource, and cloaks the true production behaviors behind artificially legible layer of metaphoric abstraction that leads consumers into a false sense of comprehension and security.

This definition is result of me trying to wrap several thousand words of thinking into one longish sentence. It is not really a layman's definition so much as a technical definition to support my analysis project.

But if you want to turn it into a simple, alarmist definition with which to scare your non-techie friends, it would be this:

The cloud is how The Man will control you ever more completely, while fostering ever more complete delusions of liberty in your head.

So far, I've applied this to computing, utilities, food production, transportation and war. It's held up.

If the analysis works out, I'll post it on my blog, http://ribbonfarm.com in the next couple of months. If I remember to do so, I'll come back here and post a link.