Here’s a tech term for you to master (again): “platform.”

You may have heard it before. The word describes an underlying technology that is a sort of stage upon which other technologies run about. The word gained currency early in the personal-computer revolution to describe PC variations. (There were the Apple platform, the IBM PC platform, the Atari platform and many others.) Later there were the dueling operating systems: the Mac platform and the Microsoft Windows platform, each upon which software and hardware was designed to run.

Now, suddenly, we’ve got online-software platforms.

Which is ironic, because the purveyors of such online “software as a service” have always boasted that it is “platform independent.” People use the software over the Internet through a Web browser, instead of running it on a computer; the selling point was that it would work equally well through a Mac or a Windows PC or a Linux machine.

But here’s Salesforce.com, which makes online customer-management software, reinventing itself as a “platform.” It’s hard to avoid the word where this blogger is today: at Dreamforce, Salesforce’s annual conference (or as they call it, a “global gathering”). Marc Benioff, the company’s CEO, used the word three times in the first minute he was on stage today. And the Business Technology Blog’s inbox is full of press releases from companies who have software designed for the new Salesforce “platform.”

It seems that every software-as-a-service company suddenly is touting itself as a platform. Facebook has taken off since it turned its social-networking site into what it calls a platform. Google isn’t calling itself a platform quite yet, but it is widely believed to be using its collection of online software to run on what would, essentially, become the Google platform.

Why do Web companies want platforms? They’ll tell you it’s because platforms create a more compelling offering for customers: Rather than just relying on software from one company, a customer can pick and choose from a collection of software that all works nicely together.

But we suspect there’s another reason: It’s a lot harder to stop using a platform. That’s a lesson tech companies have learned from Microsoft, whose Windows platform is ubiquitous. The irony is that one of the initial sales pitches of Salesforce was its platform independence; in theory, that made it easy for a corporate user to switch away from Salesforce. Such flexibility is great for a company that’s trying to figure out if it wants to use a product. But it’s bad for the company selling the stuff. When you get someone hooked on a platform, they’re pretty much stuck.