Sustainability is the intersection of the 3 E's: environment, equity, and economy. All three of these concepts must be considered equally, from beginning to end to deem a project, initiative, or product sustainable.
The most common definition of sustainabiltiy is from the Brundtland Report (1987), "Sustainable development is development that meets the needs of the present without compromising the ability of future generations to meet their own needs." Sustainability can mean different things to different people, there is no one right definition. Sustainability is also a process and we are all are in the learning process to becomming more sustainable in our individual lives and community. Because sustainability is so broad, we all have the opportunity to find an aspect of sustainabitliy that speaks to our passions, interests, life, and career.