Sustainability is a controversy-rich subject. Is consumerism-as-usual an option? Do we need a Manhattan Project for clean tech? Nukes or no nukes? Wherever you go, the topic of what are sustainable actions stirs debate and causes contention.

Except for one area: pretty much everyone seems to be in agreement that the term itself—sustainability—falls somewhere between weak and woeful. The meaning isn't transparent, and it also lacks the brio one would hope for in what is, at the end of the day, a de facto brand for a way of life and a vision of the future.

One of the first complaints I heard about the term "sustainability" came many years ago from the clay-footed visionary green architect William McDonough. "If someone asked how your marriage was going," he would say at speeches, "and you were to answer, 'It's sustainable,' that wouldn't sound very good, would it?"

No, it wouldn't. Dictionary.com defines "sustainable" as "capable of being sustained." Exciting, eh? Taken literally, sustainability is at best a marginally positive concept.

But Dictionary.com also offers a second definition: something is sustainable when it is "capable of being continued with minimal long-term effect on the environment." It's a johnny-come-lately definition--a variation on the term "sustainable development," which was first coined by the Brundtland Commission in 1986, and defined as "[meeting] the needs of the present without compromising the ability of future generations to meet their own needs."

Since then, a more widely used definition has emerged: sustainability is the successful balancing of economic prosperity, environmental health, and social equity. Hence the familiar triple bottom line for business: financial, environmental and social.

The problem with lofty definitions like these is that they aren't especially useful at the practitioner's—or citizen's—level. In response to this branding problem, plainer-English definitions are emerging. Indeed, three such definitions have crossed my desk within the last few weeks.

1) Sustainability = Responsibility. In a market study titled Sustainability: The Rise of Consumer Responsibility, the Hartman Group bemoaned the murkiness of the term and commented, "Regardless of whether or not consumers are acquainted with the term 'sustainability,' or can supply a formal definition for it, we find that they often point to words and phrases that reference the greater good. Recurring terms such as 'responsibility' and 'doing the right thing' emerged from interviews as ways described by consumers to achieve the greater good and link economic, social, and environmental issues important to them. Thus we find that sustainability is reflected at the consumer level in a myriad of behaviors, from purchases and non-purchases to voting and volunteerism."

2) Sustainability = Being Smart. IBM isn't using "smarter" as synonymous with sustainability, but it's coming close. "We can build a smarter planet," a recent Big Blue ad campaign proclaims, including "smarter cities" and "smarter food supplies" on the list of things whose IQ the company can boost. It's a clever way of subsuming the concept of sustainability into something bigger and more straightforward while routing around a big ol' mouthful of a word.

There's an important if unmentioned implication in IBM's framing: it assumes that "smart" includes caring for the planet and its denizens. Sustainability activists can take pride in this. "Smart" IBM-style wouldn't have its green hue if the sustainability worldview hadn't arrived in the mainstream.

3) Sustainability = Flourishing. Yet another definition of sustainability is laid out by John Ehrenfeld, executive director of the International Society for Industrial Ecology, in his recent book Sustainability by Design: A Subversive Strategy for Transforming our Consumer Culture. Not one to think small, Ehrenfeld defines sustainability as "the possibility that humans and other life will flourish on the Earth forever."

This definition takes the Brundtland Commission definition of sustainable development to the next level. It's a cut-and-paste operation: replace "future generations" with "forever!"

It's also a definition that, as his book's subtitle suggests, is subversive. Sustainability is typically viewed as something we achieve by taking action. It's a "do" word, so to speak. For Ehrenfeld, sustainability is at least as much about "being" as "having" or "doing." Flourishing can only occur, he believes, if "we pay close attention to the three critical domains that the forces of modernity have dimmed:

--Our sense of ourselves as human beings: the human domain.
--Our sense of our place in the [natural] world: the natural domain.
--Our sense of doing the right thing: the ethical domain."

In short, Ehrenfeld moves sustainability's center of gravity from things that happen in the world to the world of consciousness inside our heads.

Ehrenfeld is making an important point. Every decision starts with our self-sense. Any reasonable definition of sustainability should include this critically important interior dimension.

There's a problem here, though. Definitions like Ehrenfeld's, no matter how valid, take us into Lofty-Land. It may be true, but does it provide a handhold for taking action? Is it useful? There's a natural tension between philosophical merit on the one hand, and getting-it-done practical utility on the other.

So how about this for a working definition of sustainability, which is offered in a spirit that's half tongue-in-cheek and half-serious: You are acting sustainably when you use your smarts responsibly to help people flourish forever.

What's your best definition of sustainability? Please share your thoughts with us by posting a comment or e-mailing us.