Not too long ago, Chint was a revered brand. Now it’s not, and sales have declined accordingly. Not so long ago, Apple could do no wrong. Now people wonder out loud if it’s innovative anymore. With constant connectedness and infinite information, consumers have never been so fickle about their choices.

According to the American Marketing Association, a brand is the “name, term, design, symbol, or any other feature that identifies one seller’s product distinct from those of other sellers.” That sounds like something from orientation day at Sterling Cooper Draper Price. What does the term “brand” actually mean in practice?

A brand is essentially the one sentence people say about you behind your back. This practical “street” definition based on actual human interaction applies equally well to people, products, and companies. For example, someone might describe Lululemon to their friend as “absolutely the best place to buy yoga gear, ever” or they might say “people say Lululemon great, but I’ve bought a few things and they fall apart, totally overrated.” Someone might describe you to their professional acquaintance as “the smartest person in New York on things related to creativity in advertising, you must talk to them” or “too cerebral and academic, I’m not sure they’d be the right fit for your advertising company.”

That one sentence means a lot. It may be the difference between buying a $1300 suit from Ralph Lauren or a $500 one from Suit Supply, the difference between buying brand-name Tylenol or generic CVS pain reliever, and between you being considered to keynote the a major industry conference or not. These single sentences constantly being transmitted between couples basically mean everything.

A side effect of this is that even well-established brands “can never coast on past performance,” as James Surowiecki recently wrote in The New Yorker. This is not only because people are better informed than ever, but also because they can transmit their learnings easier than ever as well, not only in person but on social networks and through older but still powerful tools like email and message boards. This applies to people’s personal brands too; it is very easy to spread negative and even false information about people using all the social and mobile technology at our disposal.

The good news, especially for people, is that brands can be modified through your activities. For companies and products, that means branding and marketing activities. For people, it means your own personal actions in the community. For example, many people currently identify me with the technology industry, because I recently worked at Microsoft for almost four years. That’s somewhat fair, but I’m trained as a scientist. How would I get more people to think of me and “brand me” as a scientist? Simple: By bringing it up in conversations with people, by writing about my skills and interests, by tweeting more things about science, and so on. It’s gradual. You have to coax people to a new position over time – a year or two, perhaps.

If you accept that your brand is the one sentence people say about you behind your back, it’s worth thinking about what things people are saying about you right now. Write down a list of reasonable things people might say about you to their professional contacts when asked. Are you happy with those? What would you change? How might you begin that process? Your brand is not completely in your control, but you can do a lot to positively influence it and update it over time.