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 based on our lived expereince. Sustainability is also a process, not a destination. We are all on our path of learning to becomming more sustainable in our individual lives and as a community. As sustainability is so broad with many intersecting issues, we all have the opportunity to find an aspect of sustainability that speaks to our passions, interests, life, and career.