Carbon neutral, you may remember, was the word of the year back in 2006, but as my friend Joel Makower (executive editor of greenbiz.com, aka the guru of green business) has written, no one knows exactly what it means or even how to define a company’s carbon footprint.

So when Dell announced today that the company had become carbon neutral, I decided to take a closer look in my Sustainability column at fortune.com and cnnmoney.com. Here’s how the column begins:

Dell is announcing Wednesday that it has become carbon neutral by turning out the lights in its offices, buying wind power and protecting endangered forests in Madagascar.

It’s all part of CEO Michael Dell’s commitment to make the company that he started back in 1984 “the greenest technology company on the planet.”

But what, exactly, does becoming carbon neutral mean?

It turns out that there’s no agreed-upon definition of carbon neutral, even as rock groups like the Rolling Stones, events like the Super Bowl and the Oscars, and a growing number of companies have set carbon neutrality as a goal.

You can read the rest here.