You could use the Brundtland Commission definition (paraphrased): Meeting the needs of the present without inhibiting future generations to meet their own needs.
To me it means not depleting and even restoring the natural capital of the ecosystem. For a store selling food it might mean selling sustainable products in a sustainable way: biodegradable and/or compostable packaging, 100% renewable energy, in-fill development sites, only organic, local whenever possible, no waste generated from operations, etc...
I like Brian's description. We went to Bishop Museum the other night, and I was again reminded about how our host culture had a sustainable system supporting hundreds of thousands of people. A system that respected the land, the water, the food, the people and put everything into a harmonious balance. So, when I think of sustainable, I think the ahupua'a is a great model for our thinking.
Sustainable Living to me have so many meanings. In environmental terms, it means consuming less resources than what's available, and to use our natural resources in a healthy way so that it can naturally replenish itself. In a broader lifestyle sense, it means living a life that you can sustain/carry without burning out and having a breakdown in the middle of the day. So, having a balanced plate. This includes spending your money in a sustainable way and (as much as possible) not spending more than you have (and if you do, don't worry, just make sure you have a payoff plan).
I like how Joannie expanded to include more personal sustainability. It certainly is so important to maintain a personal balance financially, emotionally, spiritually. Although it does seem that some people like to ride life's ups and downs a little more than others, and they may not resonate with calm, smooth sailing in life.